A
Acidification｜酸性化
Volumes: V3, V4
A decrease in pH caused by the addition or production of acids. Dissolved CO₂ can acidify groundwater and promote water–rock interaction.
Aeration｜曝気
Volumes: V1, V2, V6
Contact between onsen water and air. Aeration can accelerate degassing and oxidation, particularly of CO₂ and reduced sulfur species.
Alkaline｜アルカリ性
Volumes: V1, V5, V6
A condition in which water has a pH above neutral. Many Japanese simple thermal springs are mildly to strongly alkaline.
Alkaline Simple Thermal Spring｜アルカリ性単純温泉
Volumes: V5, V6
A Japanese classification for a simple thermal spring with sufficiently high pH under the official classification system.
Alkalinity｜アルカリ度
Volumes: V3, V5
The capacity of water to neutralize acid, commonly dominated by bicarbonate in natural groundwater.
Anion｜陰イオン
Volumes: V1–V5
A negatively charged ion, such as Cl⁻, HCO₃⁻, or SO₄²⁻.
Aragonite｜アラゴナイト
Volumes: V3
A crystalline form of calcium carbonate, CaCO₃, that may precipitate from mineral-rich onsen water.
Artesian Flow｜自噴
Volumes: V1
Natural upward discharge of groundwater from a well due to subsurface pressure without mechanical pumping.
Artificial Carbonated Bath｜人工炭酸泉・人工炭酸浴
Volumes: V3, V6
Bathwater in which CO₂ is artificially dissolved to produce conditions resembling naturally carbonated spring water.
B
Bathing Condition｜入浴コンディション
Volumes: V6
The bather’s physical and situational state at the time of bathing, including fatigue, hydration, sleep, skin condition, and ambient temperature.
Bathing Load｜入浴負荷
Volumes: V6
A practical concept used in this series for the combined physiological burden produced by water temperature, bathing duration, spring chemistry, and the condition of the bather.
Bathwater｜浴槽水
Volumes: V6
The water actually present in a bath. Its chemistry may differ substantially from that of the analyzed source water.
Bicarbonate Ion (HCO₃⁻)｜炭酸水素イオン
Volumes: V1, V3, V5
A major dissolved inorganic carbon species in many groundwater and onsen systems.
Bicarbonate Spring｜炭酸水素塩泉
Volumes: V3, V6
A therapeutic spring characterized by bicarbonate as an important anion in its dissolved mineral composition.
Binary-Cycle Power Generation｜バイナリー発電
Volumes: V4
A geothermal power system in which geothermal heat vaporizes a secondary working fluid with a lower boiling point than water.
Biofilm｜バイオフィルム
Volumes: V3, V6
A community of microorganisms attached to a surface and embedded in a protective extracellular matrix.
Brine｜かん水・高塩分地層水
Volumes: V4
Highly saline subsurface water, commonly associated with sedimentary basins and formation-water systems.
Br/Cl Ratio｜Br/Cl比
Volumes: V3, V4
The bromide-to-chloride ratio, used as a geochemical tracer for investigating the origin and evolution of saline waters.
Bubble Nucleation｜気泡核形成
Volumes: V3
The initial formation of gas bubbles from dissolved gas, often promoted by microscopic irregularities on surfaces.
Buoyancy｜浮力
Volumes: V6
The upward force exerted by water on an immersed body.
C
Calcium Carbonate (CaCO₃)｜炭酸カルシウム
Volumes: V3, V4
A mineral-forming compound that may precipitate when bicarbonate-rich water loses CO₂.
Calcium Ion (Ca²⁺)｜カルシウムイオン
Volumes: V1, V3, V4
A major cation derived from mineral dissolution and commonly involved in carbonate precipitation.
Cap Rock｜キャップロック
Volumes: V4
A relatively impermeable geological layer that restricts the upward movement of hydrothermal fluids.
Carbon Dioxide (CO₂)｜二酸化炭素
Volumes: V3, V6
A gas that dissolves in groundwater, acidifies water, promotes water–rock reaction, and can cross the skin under suitable bathing conditions.
Carbon Dioxide Spring｜二酸化炭素泉
Volumes: V3, V6
A Japanese therapeutic-spring category characterized by a specified concentration of free dissolved CO₂.
Carbon Isotopes｜炭素同位体
Volumes: V3
Isotopes of carbon used to investigate the origin of dissolved or gaseous carbon in natural fluids.
Carbonate Ion (CO₃²⁻)｜炭酸イオン
Volumes: V3
A dissolved inorganic carbon species favored under relatively alkaline conditions.
Carbonate System｜炭酸系
Volumes: V3
The chemical system linking dissolved CO₂, bicarbonate, and carbonate in water.
Cation｜陽イオン
Volumes: V1–V5
A positively charged ion such as Na⁺, Ca²⁺, or Mg²⁺.
Cation Exchange｜陽イオン交換
Volumes: V3, V4
Exchange between dissolved cations and cations held on clay minerals or other reactive mineral surfaces.
Charge Balance｜電荷収支
Volumes: V1, V3
The requirement that the total positive and negative electrical charges in a water sample remain approximately balanced.
Chloride Ion (Cl⁻)｜塩化物イオン
Volumes: V1, V4
A major anion that is relatively conservative in many groundwater systems and useful as a tracer of fluid origin and mixing.
Chloride Spring｜塩化物泉
Volumes: V4, V6
A therapeutic spring in which chloride is a major anion.
Colloid｜コロイド
Volumes: V2–V4
Very fine particles dispersed in water that may strongly influence color and light scattering.
Concentration Dependence｜濃度依存性
Volumes: V6
Variation in a physical, chemical, or biological response as the concentration of a substance changes.
Connate Water｜同生水
Volumes: V3, V4
Water trapped within sediments during or soon after deposition. It is related to, but should not automatically be equated with, fossil seawater.
Context Effect｜コンテクスト効果
Volumes: V6
Physiological or psychological responses influenced by setting, expectation, environment, and experience in addition to any specific chemical action.
Contrast Bathing｜温冷交代浴
Volumes: V3, V6
Alternating immersion in warm or hot water and cooler water.
Crustal CO₂｜地殻起源CO₂
Volumes: V3
Carbon dioxide produced within crustal rocks or sediments rather than directly from the mantle.
Cutaneous Blood Flow｜皮膚血流
Volumes: V3, V6
Blood flow through vessels supplying the skin.
D
Deep Fluid｜深部流体
Volumes: V1, V3, V4
Fluid originating or circulating at substantial depth in the crust and capable of transporting heat, gases, and dissolved chemical components upward.
Deep Groundwater｜深層地下水
Volumes: V1, V4
Groundwater circulating at greater depth than ordinary shallow groundwater.
Degassing｜脱ガス
Volumes: V1–V4, V6
Loss of dissolved gases from water, commonly caused by decreasing pressure, increasing temperature, or contact with air.
Dilution｜加水・希釈
Volumes: V1, V6
Reduction in dissolved-component concentration by mixing source water with additional water.
Direct In-Bath Spring Emergence｜足元湧出
Volumes: V6
A system in which onsen water emerges naturally directly into or beneath the bathing pool.
Discharge Rate｜湧出量
Volumes: V1
The volume of spring water discharged per unit time.
Dissociation｜解離
Volumes: V2, V3
Separation of a dissolved chemical species into ions or other species according to chemical equilibrium.
Dissolved CO₂｜溶存二酸化炭素
Volumes: V3
Carbon dioxide dissolved in water.
Dissolved Inorganic Carbon (DIC)｜溶存無機炭素
Volumes: V3
The total dissolved inorganic carbon present mainly as dissolved CO₂, HCO₃⁻, and CO₃²⁻.
Dissolved Organic Matter (DOM)｜溶存有機物
Volumes: V5
Organic compounds dissolved in natural water.
Drilled Onsen｜掘削泉
Volumes: V1
An onsen obtained by drilling a well into a subsurface thermal or mineral-water reservoir.
E
Elemental Sulfur (S⁰)｜元素硫黄
Volumes: V2
Sulfur in its elemental oxidation state, often occurring as fine yellow or whitish particles in sulfur-rich waters.
Epidermis｜表皮
Volumes: V6
The outer cellular layer of the skin.
Eh / Redox Potential｜酸化還元電位
Volumes: V2, V6
An electrochemical measure reflecting the tendency of a solution toward oxidation or reduction.
Evidence｜科学的エビデンス
Volumes: V6
Empirical scientific information used to support, reject, or qualify a proposed effect or mechanism.
Excess Sodium｜余剰Na
Volumes: V3
Sodium present in excess of that expected from a simple Na–Cl relationship, used as a conceptual clue when investigating bicarbonate-rich waters.
F
Fault｜断層
Volumes: V1, V3–V5
A fracture zone along which displacement has occurred and which may provide a pathway or barrier for subsurface fluids.
Formation Water｜地層水
Volumes: V3, V4
Water contained within the pore spaces of sedimentary or other geological formations.
Fossil Seawater｜化石海水
Volumes: V3, V4
Ancient seawater or seawater-derived formation water preserved underground and generally modified by long-term geological and geochemical processes.
Free Carbon Dioxide｜遊離二酸化炭素
Volumes: V3
Dissolved molecular CO₂ not bound as bicarbonate or carbonate.
Free-Flowing Source Water｜源泉かけ流し
Volumes: V1, V6
Onsen water continuously supplied from the source and discharged rather than routinely recirculated.
Fulvic Acid｜フルボ酸
Volumes: V5
A relatively soluble fraction of humic substances occurring in organic-rich natural waters.
Fumarole｜噴気孔
Volumes: V4
An opening through which volcanic or hydrothermal gases and steam escape to the surface.
G
General Indications｜一般適応症
Volumes: V6
Indications associated broadly with bathing rather than specifically with one spring-water composition.
Geochemical Thermometer｜地球化学温度計
Volumes: V4
A chemical relationship used to estimate subsurface temperatures from the composition of geothermal fluids.
Geothermal Power Generation｜地熱発電
Volumes: V4
Production of electricity from heat stored in the Earth.
Geothermal Reservoir｜地熱貯留層
Volumes: V4
A permeable subsurface rock body containing hot water or steam.
Granite｜花崗岩
Volumes: V5
A silica-rich intrusive igneous rock relevant to groundwater alkalinity and natural radioelement distributions.
Granodiorite｜花崗閃緑岩
Volumes: V5
An intrusive igneous rock chemically intermediate between granite and diorite.
Gravity Anomaly｜重力異常
Volumes: V5
A deviation of measured gravity from a reference value, reflecting variations in subsurface density.
Groundwater｜地下水
Volumes: V1–V5
Water occupying pores and fractures beneath the ground surface.
Groundwater Mixing｜地下水混合
Volumes: V3, V4
Mixing between groundwater bodies of different origins or chemical compositions.
H
Half-Life｜半減期
Volumes: V5, V6
The time required for half of a radioactive population to decay.
Heat-Shock Proteins (HSPs)｜ヒートショックプロテイン
Volumes: V6
Cellular proteins whose expression may change in response to heat and other stresses.
Heating｜加温
Volumes: V1, V6
Artificial increase of source-water temperature before or during bathing.
Helium Isotopes｜ヘリウム同位体
Volumes: V3
Isotopic ratios, especially involving ³He and ⁴He, used to investigate mantle and crustal contributions to geothermal fluids.
Henry’s Law｜ヘンリーの法則
Volumes: V3
A relationship describing how the amount of a dissolved gas depends on its partial pressure above the liquid, under specified conditions.
Hidden Sulfur Spring｜隠れ硫黄泉
Volumes: V5, V6
A descriptive term used in this series for water containing sulfur below an official spring-classification threshold but displaying observable sulfur-related characteristics. Not an official Japanese spring classification.
High-Salinity Formation Water｜高塩分地層水
Volumes: V3, V4
Formation water containing high concentrations of dissolved salts.
Hot Spring Law｜温泉法
Volumes: V1
Japan’s legal framework governing the definition, protection, extraction, and utilization of onsen resources. The official legal and analytical definitions should be distinguished from popular usage.
Humic Acid｜フミン酸
Volumes: V5
A fraction of humic substances produced through transformation of organic matter.
Humic Substances｜腐植物質
Volumes: V5
Complex organic substances formed during decomposition and alteration of biological material.
Hydrostatic Pressure｜静水圧
Volumes: V6
Pressure exerted by water on an immersed object or body.
Hydrothermal Alteration｜熱水変質
Volumes: V4
Mineralogical and chemical alteration of rock caused by interaction with hot aqueous fluids.
Hydrothermal System｜熱水系
Volumes: V4
A subsurface system in which heated water or other fluids circulate through permeable rocks.
Hydrogen Sulfide (H₂S)｜硫化水素
Volumes: V2, V6
A volatile reduced sulfur compound responsible for much of the characteristic odor associated with many sulfur springs.
Hydrogen (H₂)｜水素分子
Volumes: V5, V6
Molecular hydrogen that may be naturally produced in strongly reducing water–rock reaction systems such as serpentinization.
Hydrophilicity｜親水性
Volumes: V6
Chemical affinity for water.
Hypertonic｜高張性
Volumes: V4
Having relatively high osmotic concentration under the Japanese onsen classification system.
Hypotonic｜低張性
Volumes: V4
Having relatively low osmotic concentration under the Japanese onsen classification system.
I
Indication｜適応症
Volumes: V6
A condition or circumstance for which a bathing method or spring type is officially or traditionally considered suitable. Official Japanese onsen indications are governed by national standards rather than by the spring operator alone.
Individual Variation｜個人差
Volumes: V6
Differences in physical or subjective responses among individuals exposed to similar bathing conditions.
Ion Exchange｜イオン交換
Volumes: V3, V4
Chemical exchange of ions between water and reactive mineral surfaces.
Ionic Strength｜イオン強度
Volumes: V3, V4
A measure of the total electrical environment created by dissolved ions in a solution.
Iodide Ion (I⁻)｜よう化物イオン
Volumes: V4
A dissolved form of iodine found at elevated concentrations in some organic-rich saline formation waters.
Iodine-Rich Spring｜含よう素泉
Volumes: V4
A Japanese therapeutic-spring category based on a specified iodide concentration.
Isotonic｜等張性
Volumes: V4
Having an osmotic concentration within the defined isotonic range.
L
Legionella spp.｜レジオネラ属菌
Volumes: V6
Bacteria capable of proliferating in engineered warm-water systems under suitable conditions and causing Legionella infection.
Light Scattering｜光散乱
Volumes: V2–V4
Redirection of light by particles or microscopic variations in a fluid, contributing to white, blue, or turquoise appearance.
Lipid Removal / Defatting｜脱脂
Volumes: V6
Removal of lipid material from the surface of the skin.
Lipophilicity｜親油性
Volumes: V6
Chemical affinity for lipids or nonpolar environments.
M
Magmatic CO₂｜マグマ起源CO₂
Volumes: V3
Carbon dioxide released from magma or magmatic fluids.
Magmatic Fluid｜マグマ性流体
Volumes: V3, V4
A volatile-rich fluid separated directly or indirectly from magma.
Magnesium Ion (Mg²⁺)｜マグネシウムイオン
Volumes: V1, V3, V4
A common dissolved cation useful in interpreting water–rock interaction and seawater-derived fluids.
Mantle-Derived CO₂｜マントル起源CO₂
Volumes: V3
Carbon dioxide ultimately derived from the Earth’s mantle.
Marine Sediment｜海成堆積物
Volumes: V2–V5
Sediment deposited in a marine environment.
Meteoric Water｜天水
Volumes: V1, V3, V4
Water derived from precipitation that infiltrates underground.
Metasilicic Acid｜メタケイ酸
Volumes: V4, V6
The conventional Japanese onsen-analysis expression for dissolved silica reported in terms of H₂SiO₃. Its analytical presence should not by itself be treated as proof of a specific skin effect.
Microbially Induced Carbonate Precipitation｜微生物誘導炭酸塩沈殿
Volumes: V3
Carbonate precipitation promoted or mediated by microbial activity.
Mineral Precipitate｜鉱物析出物
Volumes: V1–V4
Solid mineral material formed from dissolved components in water.
Moor Spring｜モール泉
Volumes: V5
An informal term for amber to dark-colored thermal or groundwater rich in dissolved organic substances. Not a formal Japanese therapeutic-spring category.
N
Na/Cl Ratio｜Na/Cl比
Volumes: V3, V4
The sodium-to-chloride ratio used to investigate mixing and geochemical modification of saline waters.
Natural Carbonated Spring｜天然炭酸泉
Volumes: V3
Naturally occurring groundwater or onsen containing abundant dissolved carbon dioxide.
Natural Discharge｜自然湧出
Volumes: V1
Natural emergence of groundwater at the surface without pumping.
Neutralization｜中和反応
Volumes: V3
Chemical consumption of acidity or alkalinity through reaction.
Nitric Oxide (NO)｜一酸化窒素
Volumes: V3, V6
A biological signaling molecule involved in regulation of vascular tone.
Nucleation｜核形成
Volumes: V3
The initial formation of a new phase, such as a gas bubble or mineral crystal.
O
Olivine｜かんらん石
Volumes: V5
A magnesium-iron silicate mineral common in ultramafic rocks.
Onsen｜温泉
Volumes: V1–V6
A Japanese term for naturally occurring hot or mineral water—and, legally, certain naturally occurring steam and gases—meeting conditions defined by Japan’s Hot Spring Law. It should not simply be translated as “hot water,” because legally recognized onsen may be cooler than 25°C if compositional criteria are satisfied.
Onsen Analysis Report｜温泉分析書
Volumes: V1–V6
An analytical report describing the physical and chemical characteristics of sampled onsen water.
Onsen Source｜源泉
Volumes: V1, V6
The original spring, well, or other source from which onsen water or fluid is obtained.
Onsen Water Management｜湯使い
Volumes: V6
How source water is transported, stored, cooled or heated, diluted, circulated, filtered, disinfected, and supplied to the bath. In this series, yu-zukai may be retained as a Japanese cultural and technical term.
Oxidation｜酸化
Volumes: V2, V6
A chemical process involving loss of electrons or an increase in oxidation state.
Oxidation State｜酸化数
Volumes: V2
A formal representation of the electron state of an element within a chemical species.
Oxidative Stress｜酸化ストレス
Volumes: V6
A biological condition involving imbalance between oxidative processes and antioxidant defenses.
Oxidation–Reduction Potential (ORP)｜酸化還元電位
Volumes: V2, V6
An electrochemical property of water reflecting redox conditions. A low ORP should not automatically be equated with physiological “antioxidant power.”
P
Partial Pressure｜分圧
Volumes: V3
The pressure contributed by an individual gas component within a gas mixture.
Percutaneous Absorption｜経皮吸収
Volumes: V6
Movement of a chemical substance through the skin.
Peridotite｜かんらん岩
Volumes: V5
An ultramafic rock composed largely of olivine and pyroxene.
pH｜pH
Volumes: V1–V6
A logarithmic measure related to hydrogen-ion activity and therefore to the acidic or alkaline character of water.
Phase Separation｜相分離
Volumes: V4
Separation of a hydrothermal fluid into liquid-rich and vapor-rich phases.
Plagioclase Feldspar｜斜長石
Volumes: V5
A common rock-forming mineral that can participate in groundwater–rock reactions.
Pore Water｜間隙水
Volumes: V4
Water occupying pore spaces between sediment or rock particles.
Pre-Bath Self-Check｜入浴前セルフチェック
Volumes: V6
A simple assessment of the bather’s current condition before deciding how to bathe.
Precipitation｜析出・沈殿
Volumes: V2–V4
Formation of a solid phase from dissolved chemical components.
R
Radiation Hormesis｜放射線ホルミシス
Volumes: V6
The hypothesis that low-dose ionizing radiation may trigger beneficial biological responses. It should be presented as a scientific hypothesis rather than as a universally established therapeutic mechanism.
Radioactive Spring｜放射能泉
Volumes: V5, V6
A Japanese therapeutic-spring category based on radon concentration.
Radium (Ra)｜ラジウム
Volumes: V5
A radioactive element in the uranium decay series and a parent source of radon.
Radon (Rn)｜ラドン
Volumes: V5, V6
A radioactive noble gas generated naturally by radioactive decay of radium.
Recirculation｜循環
Volumes: V1, V6
Return of used bathwater to the bathing pool through a circulation system.
Recirculation and Filtration｜循環ろ過
Volumes: V1, V6
Reuse of bathwater after passage through a filtration system.
Redox｜酸化還元
Volumes: V2, V5, V6
Chemical processes involving electron transfer and changes in oxidation state.
Reduction｜還元
Volumes: V2, V6
A chemical process involving gain of electrons or decrease in oxidation state.
Residence Time｜滞留時間
Volumes: V1, V2, V4, V5
The length of time water or fluid remains within a particular subsurface or engineered system.
S
Salinity｜塩分
Volumes: V4
The concentration of dissolved salts in water.
Saline Formation Water｜塩分に富む地層水
Volumes: V3, V4
Formation water containing substantial dissolved salts.
Scale｜スケール
Volumes: V4
Mineral deposits that accumulate inside pipes, wells, heat exchangers, and geothermal equipment.
Sedimentary Basin｜堆積盆
Volumes: V3–V5
A geological depression in which sediment accumulates over long periods.
Serpentinization｜蛇紋岩化作用
Volumes: V5
Hydration and alteration of ultramafic rock, capable of generating highly alkaline water and molecular hydrogen.
Silica｜シリカ
Volumes: V4
Silicon dioxide-related material occurring in dissolved, colloidal, or solid forms in geothermal systems.
Siliceous Sinter｜珪華
Volumes: V4
Silica-rich deposits precipitated from geothermal or hot-spring water.
Simple Carbon Dioxide Spring｜単純二酸化炭素泉
Volumes: V5
A simple thermal spring that also meets the relevant criterion for carbon dioxide.
Simple Sulfur Spring｜単純硫黄泉
Volumes: V5
A simple thermal spring that also meets the relevant sulfur criterion.
Simple Thermal Spring｜単純温泉
Volumes: V1, V5
A Japanese therapeutic-spring category defined primarily by temperature and relatively low total dissolved matter under the official analytical classification.
Skin｜皮膚
Volumes: V6
The body’s major external organ and the principal interface between the bather and onsen water.
Skin Barrier｜皮膚バリア
Volumes: V6
Structures and functions, particularly those of the stratum corneum, that limit water loss and penetration of external substances.
Sodium Hypochlorite｜次亜塩素酸ナトリウム
Volumes: V6
A chlorine-containing compound commonly used for disinfection of bathing-water systems.
Sodium Ion (Na⁺)｜ナトリウムイオン
Volumes: V1, V3, V4
A major cation in many groundwater, bicarbonate, and chloride spring systems.
Solubility｜溶解度
Volumes: V2–V4
The amount of a substance that can dissolve under specified temperature, pressure, and chemical conditions.
Source Temperature｜源泉温度
Volumes: V1–V6
The temperature of water measured at or near its source.
Source-Water Freshness｜源泉鮮度
Volumes: V2, V6
A practical concept in this series describing how little the source water’s chemical character has changed before it reaches the bather.
Spring-Type-Specific Indications｜泉質別適応症
Volumes: V6
Official or traditionally recognized indications associated with particular spring-water classifications.
Stable Isotopes｜安定同位体
Volumes: V3, V4
Non-radioactive isotopes used to investigate the source, history, and mixing of natural waters.
Stratum Corneum｜角層
Volumes: V6
The outermost layer of the epidermis and a major component of the skin barrier.
Sulfate Ion (SO₄²⁻)｜硫酸イオン
Volumes: V1–V4
A major anion representing a highly oxidized form of sulfur.
Sulfate Reduction｜硫酸還元
Volumes: V2–V4
Reduction of sulfate to reduced sulfur species, commonly associated with microbial metabolism in oxygen-poor environments.
Sulfate-Reducing Microorganisms｜硫酸還元微生物
Volumes: V2
Microorganisms that use sulfate in anaerobic metabolism and can generate sulfide.
Sulfide Mineral｜硫化鉱物
Volumes: V2
A mineral containing sulfur in reduced form, often combined with metals.
Sulfur｜硫黄
Volumes: V2
A chemical element occurring in multiple oxidation states and chemical forms in hydrothermal systems.
Sulfur Spring｜硫黄泉
Volumes: V2, V6
A Japanese therapeutic-spring category defined by the concentration of specified reduced sulfur species.
Supercritical CO₂｜超臨界CO₂
Volumes: V3
Carbon dioxide above its critical temperature and pressure, where distinct liquid and gas phases cease to exist.
Supercritical Water｜超臨界水
Volumes: V4
Water above its critical temperature and pressure, where the normal liquid–vapor distinction disappears.
Supersaturation｜過飽和
Volumes: V3, V4
A state in which a dissolved component exceeds its equilibrium solubility and may precipitate or exsolve.
T
Therapeutic Mineral Spring｜療養泉
Volumes: V1–V6
An onsen water meeting the Japanese criteria for temperature and/or specified chemical components for therapeutic use. The official classification is narrower than the general legal definition of onsen.
Thermal Effect｜温熱作用
Volumes: V6
Physiological responses produced by heat transferred from bathwater to the body.
Thermal Water｜温水・温泉水
Volumes: V1
Groundwater naturally warmed by subsurface geological processes.
Threshold Value｜基準値
Volumes: V2, V3, V5, V6
A defined concentration or physical value used for classification or for evaluating a particular phenomenon.
Thiosulfate Ion (S₂O₃²⁻)｜チオ硫酸イオン
Volumes: V2
An intermediate sulfur oxyanion that may occur during oxidation of reduced sulfur.
Total Dissolved Components｜溶存成分総量
Volumes: V1, V4, V5
The total quantity of dissolved substances reported in an onsen analysis under the applicable analytical convention.
Total Sulfur｜総硫黄
Volumes: V2
A collective measure of specified reduced sulfur species used in Japanese onsen analysis and classification.
Transcutaneous CO₂ Absorption｜二酸化炭素経皮吸収
Volumes: V3, V6
Movement of dissolved CO₂ through the skin.
Transepidermal Water Loss (TEWL)｜経皮水分蒸散
Volumes: V6
Passive movement of water through the epidermis to the surrounding air.
Travertine｜トラバーチン
Volumes: V3
A terrestrial carbonate deposit formed by precipitation from mineral-rich spring water.
U
Ultramafic Rock｜超苦鉄質岩
Volumes: V5
Mg- and Fe-rich rock containing relatively little silica and commonly dominated by minerals such as olivine and pyroxene.
Uranium Decay Series｜ウラン壊変系列
Volumes: V5
A sequence of radioactive transformations beginning with uranium and ultimately producing stable lead, with radium and radon as intermediate products.
V
Vasodilation｜血管拡張
Volumes: V3, V6
Widening of blood vessels, resulting in increased local blood flow.
Volcanic Front｜火山フロント
Volumes: V2–V4
A geographical line or zone marking the arcward limit of active volcanism in a subduction-zone volcanic arc.
Volcanic Gas｜火山ガス
Volumes: V2–V4
Gas released from magma or volcanic-hydrothermal systems, commonly including H₂O, CO₂, SO₂, H₂S, and other species.
Volcanic Hydrothermal Fluid｜火山性熱水
Volumes: V4
Hot subsurface fluid associated with volcanic heat and volcanic-hydrothermal systems.
Volcanic Onsen｜火山性温泉
Volumes: V1–V4
An onsen whose heat, fluid chemistry, or gases are substantially influenced by volcanic or magmatic processes.
W
Water–Rock Interaction｜水―岩石反応
Volumes: V1–V5
Chemical reactions between groundwater or hydrothermal fluid and surrounding rocks and minerals. One of the central concepts of the entire series.
Water Analysis｜水質分析
Volumes: V1–V6
Measurement of physical and chemical properties of water.
Y — Japanese Onsen Terms Worth Keeping in English
Netsu-no-yu｜熱の湯
Volumes: V4, V6
Literally “warming bath.” A traditional Japanese expression particularly associated with chloride springs. The physiological basis is examined separately in Volume 6.
Onsen｜温泉
Volumes: V1–V6
The central Japanese term of the series. Retaining onsen rather than translating every occurrence as hot spring avoids confusion with cool mineral springs recognized under Japanese law.
Rotenburo｜露天風呂
Volumes: V1, V6
An outdoor bathing pool supplied with onsen or other bathwater.
Shio-yu｜潮湯
Volumes: V4
A traditional bath using seawater or salt-rich water.
Yu-no-hana / Yunohana｜湯の花
Volumes: V1–V5
Mineral or other natural precipitates formed from onsen water. The material may consist of sulfur, carbonate, silica, iron compounds, or mixtures depending on the spring.
Yu-zukai｜湯使い
Volumes: V6
The complete way an onsen source is handled between its source and the bather—including transport, storage, cooling, heating, dilution, circulation, filtration, and disinfection. This series treats yu-zukai as a key concept for understanding why source-water chemistry and actual bathwater may differ.