1866, from French calorie, from Latin calor (genitive caloris) "heat," from PIE *kle-os-, suffixed form of root *kele- (1) "warm" (cognates: Latin calidus "warm," calere "be hot;" Sanskrit carad- "harvest," literally "hot time;" Lithuanian silti "become warm," silus "August;" Old Norse hlær, Old English hleow "warm").In scientific use, largely replaced 1950 by the joule. As a unit of energy, defined as "heat required to raise 1 gram of water 1 degree Celsius" (the small or gram calorie), but also as "heat required to raise 1 kilogram of water 1 degree Celsius" (the large calorie or kilocalorie).