The prefix thermo- comes from Greek thermos, meaning “warm, hot.” When used to form words in English, thermo- generally refers to heat, as in thermodynamic, or sometimes to thermoelectricity, as in thermocouple. Most of the words that begin with thermo-, such as thermodynamics, thermoelectricity, thermostat, and thermosphere, have only come into being in the 19th and 20th centuries. Sometimes before a vowel thermo- becomes therm-, as in thermanesthesia, which means “inability to feel hot or cold.”
