A calorie is a unit of measurement for energy. Calorie is French and derives from the Latin calor (heat). In most fields, it has been replaced by the joule, the SI unit of energy. However, the kilocalorie or Calorie (capital "C") remains in common use for the amount of food energy.
In scientific contexts, the name "calorie" refers strictly to the gram calorie, and this unit has the symbol cal. SI prefixes are used with this name and symbol, so that the kilogram calorie is known as the "kilocalorie" and has the symbol kcal. In non-scientific contexts the kilocalorie is often referred to as a Calorie (capital "C"), or just a calorie, and it has to be inferred from the context that the small calorie is not intended.
The conversion factor between calories and joules is numerically equivalent to the specific heat capacity of liquid water (in SI units).