The calorie is a pre-SI unit of energy, in particular, heat. In most fields, its use is archaic, and the SI unit of energy, the joule, has become accepted. However, it remains in common use as a unit of food energy. It was first defined by Professor Nicolas Clément in 1824 as a kilogram-calorie, and this definition entered French and English dictionaries between 1841 and 1867. Etymology: French calorie, from Latin calor (heat).

The unit calorie has historically been used in two major alternate definitions that differ by a factor of 1000: