Calorie
A thermal unit of energy — the heat needed to raise one gram of water by one degree Celsius — equal to 4.186 joules.
Definition
The calorie is a unit of energy defined thermally: the small (gram) calorie is the heat required to raise one gram of water by one degree Celsius, approximately 4.186 joules. It predates the recognition that heat is energy, which is why it was originally defined through water rather than through mechanics.
Because water's specific heat varies slightly with temperature, the calorie has several precise definitions (the 15 °C calorie, the thermochemical calorie of exactly 4.184 J, and others); for most purposes 1 cal ≈ 4.186 J. The 'Calorie' on food labels, written with a capital C, is in fact a kilocalorie — 1000 small calories, about 4186 J.
Since the calorie and the joule both measure the same quantity, the calorie is now a non-SI unit retained mainly in nutrition and some chemistry; the joule is preferred in physics. The bridge between them is Joule's mechanical equivalent of heat.
History
Introduced in early-nineteenth-century calorimetry (the term is attributed to Nicolas Clément, c. 1819–24); superseded in physics by the joule once the mechanical equivalent of heat was established.