A unit of heat and energy equal to the amount of energy requires to raise the temperature of 1 gram of water by 1° Celsius. There are several flavors of calories defined. A "conventional" calorie is 4.186 J, a thermodynamic calorie (the one most commonly encountered in physics) is equal to 4.184 J, and an "International Table" calorie is equal to 4.1868 J. When written with a capital "C" ("Calorie" instead of "calorie"), the "dietary calorie" equal to 1000 usual calories is being used.

Joule, Thermodynamic Calorie