proprietary name for a type of liquid gas sold in Britain, 1936, from Latin calor, literally "heat" (from PIE root *kele- (1) "warm").
hypothetical fluid in a now-discarded model of heat exchange, 1792, from French calorique, coined in this sense by Lavoisier, from Latin calorem "heat" (nominative calor), from PIE root *kele- (1) "warm." The adjective, "pertaining to heat or the principle of heat," is recorded from 1865.
unit of heat in physics, 1866, from French calorie, from Latin calor (genitive caloris) "heat," from PIE *kle-os-, suffixed form of root *kele- (1) "warm."
As a unit of energy, defined as "heat required to raise 1 gram of water 1 degree Celsius" (the small or gram calorie), but as a measure of the energy-producing value of food, "heat required to raise 1 kilogram of water 1 degree Celsius" (the large calorie or kilocalorie). In part because of this confused definition, it was largely replaced 1950 in scientific use by the joule. Calorie-counting or -watching as a method of scientific weight-regulation is attested by 1908.