1. any of several approximately equal units of heat, each measured as the quantity of heat required to raise the temperature of 1 gram of water by 1°C from a standard initial temperature, especially from 3.98°C, 14.5°C, or 19.5°C, at 1 atmosphere pressure. also called gram calorie, small calorie.
2. the unit of heat equal to 1/100 the quantity of heat required to raise the temperature of 1 gram of water from 0 to 100°C at 1 atmosphere pressure. also called mean calorie.
3. the unit of heat equal to the amount of heat required to raise the temperature of 1 kilogram of water by 1°C at 1 atmosphere pressure. also called kilocalorie, kilogram calorie, large calorie.
4. A unit of energy-producing potential equal to this amount of heat that is contained in food and released upon oxidation by the body. also called nutritionist's calorie.

1. the gram calorie or small calorie, a non-si unit of energy, equivalent to approximately 4.2 joules. this unit was widely used in chemistry and physics, being the amount of energy needed to raise the temperature of 1 gram of water by 1 °C.
2. kilogram calorie or large calorie. A unit of energy 1000 times larger than the gram calorie. it is equivalent to the gram kilocalorie, approximately 4.2 kilojoules.