calorie (n.)

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.

Definitions of calorie
calorie (n.)
unit of heat defined as the quantity of heat required to raise the temperature of 1 gram of water by 1 degree centigrade at atmospheric pressure;
Synonyms: gram calorie / small calorie
Calorie (n.)
a unit of heat equal to the amount of heat required to raise the temperature of one kilogram of water by one degree at one atmosphere pressure; used by nutritionists to characterize the energy-producing potential in food;
Synonyms: kilogram calorie / kilocalorie / large calorie / nutritionist's calorie