The calorie is a unit of energy defined as the amount of heat needed to raise the temperature of a quantity of water by one degree.
For historical reasons, two main definitions of calorie are in wide use. The small calorie or gram calorie (usually denoted cal) is the amount of heat needed to raise the temperature of one gram of water by one degree Celsius (or one kelvin). The large calorie, food calorie, or kilocalorie (Cal, Calorie or kcal), most widely used in nutrition, is the amount of heat needed to cause the same increase in one kilogram of water. Thus, 1 kilocalorie (kcal) = 1000 calories (cal). By convention in food science, the large calorie is commonly called Calorie (with a capital C by some authors to distinguish from the smaller unit). In most countries, labels of industrialized food products are required to indicate the nutritional energy value in (kilo or large) calories per serving or per weight.
Calorie relates directly to the metric system, and therefore to the SI system. It has been regarded as obsolete within the scientific community since the adoption of the SI system, but is still in some use. The SI unit of energy is the joule. One calorie is defined as exactly 4.184 J, and one Calorie (kilocalorie) is 4184 J.