-therm Also -thermal, -thermy, -thermic, and -thermia.
[Greek thermē, heat.]
Most terms in -therm categorise animals on the basis of the way they adapt to changes in ambient temperature: an ectotherm (Greek ektos, outside) is dependent on external sources of body heat; an endotherm (Greek endon, within) is capable of internal generation of heat; a poikilotherm (Greek poikilos, variegated, varied), cannot regulate its body temperature except by its behaviour, such as basking or burrowing. A term with a separate sense is isotherm (Greek isos, equal), a line on a map connecting points having the same temperature at a given time.
Such nouns often have related adjectives in -thermal: endothermal, isothermal. Other examples are found in geology, based on the adjective thermal: geothermal, produced by the internal heat of the earth; hydrothermal, denoting the action of heated water in the earth's crust. Some adjectives are formed using -thermic: homeothermic, poikilothermic. Exothermic and endothermic have specific senses in chemistry for reactions that are accompanied respectively by the release or absorption of heat.
Nouns in -thermy are usually abstract terms for the state: ectothermy, poikilothermy, except that diathermy (Greek dia, through) is a medical technique involving the production of heat in the body by high-frequency electric currents. Some nouns have the ending -thermia, for medical states linked to temperature: hyperthermia (Greek huper, over, beyond), the condition of having a body temperature greatly above normal; hypothermia (Greek hupo, under), the reverse condition, of having an abnormally (typically dangerously) low body temperature.