Etymology
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thermal (adj.)
1756, "having to do with hot springs," from French thermal (Buffon), from Greek therme "heat, feverish heat," from PIE root *gwher- "to heat, warm." Sense of "having to do with heat" is first recorded 1837. The noun meaning "rising current of relatively warm air" is recorded from 1933.
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hydrothermal (adj.)
"of or pertaining to heated water," 1855, in geology, from hydro- "water" + thermal (adj.).
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athermanous (adj.)
"heat-resistant, impervious to radiant heat," 1839, from a- (3) "not, without" + Greek thermainein "impart heat," from thermos "hot" (see thermal).
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exothermic (adj.)

1874, in chemistry, "relating to a liberation of heat," modeled on French exothermique (1869, Berthelot); see exo- + thermal.

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hyperthermia (n.)
1878, medical Latin, from hyper- "over, exceedingly, to excess" + Greek therme "heat" (see thermal) + abstract noun ending -ia.
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isotherm (n.)
"line connecting points on the earth having the same mean temperature," 1850, from French isotherme (von Humboldt, 1817), from Greek isos "equal, identical" (see iso-) + therme "heat" (see thermal (adj.)).
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endothermic (adj.)

1869, originally in chemistry, "causing, relating to, or requiring the absorption of heat," from French endothermique (1868, Berthelot); see endo- + thermal. By 1947 in biology, "dependent on or capable of the internal generation of heat; warm-blooded."

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thermometer (n.)
1630s, from French thermomètre (1620s), coined by Jesuit Father Jean Leuréchon from Greek thermos "hot" (see thermal) + metron "measure" (from PIE root *me- (2) "to measure"). An earlier, Latinate form was thermoscopium (1610s). The earliest such device was Galileo's air-thermometer, invented c. 1597. The typical modern version, with mercury in glass, was invented by Fahrenheit in 1714. Related: Thermometric; thermometrical.
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*gwher- 

Proto-Indo-European root meaning "to heat, warm."

It forms all or part of: brand; brandish; brandy; brimstone; brindled; forceps; Fornax; fornicate; fornication; fornix; furnace; hypothermia; thermal; thermo-; Thermopylae; Thermos.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit gharmah "heat;" Old Persian Garmapada-, name of the fourth month, corresponding to June/July, from garma- "heat;" Hittite war- "to burn;" Armenian jerm "warm;" Greek thermos "warm;" Latin formus "warm," fornax "oven;" Old Irish fogeir "heated;" Old English bærnan "to kindle."

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