A calorie is a unit of measurement for energy. In most fields, it has been replaced by the joule, the SI unit of energy. However, the kilocalorie or calorie remains in common use for the amount of food energy. The calorie was first defined by Professor Nicolas Clément in 1824 as a kilogram-calorie and this definition entered French and English dictionaries between 1842 and 1867.The calorie was never an SI unit. Modern definitions for calorie fall into two classes:The small calorie or gram calorie approximates the energy needed to increase the temperature of 1 gram of water by 1 °C. This is about 4.184 joules.The large calorie or kilogram calorie approximates the energy needed to increase the temperature of 1 kg of water by 1 °C. This is about 4.184 kJ, and exactly 1000 small calories.In some scientific contexts such as physics and chemistry, the name "calorie" refers strictly to the gram calorie, and this unit has the symbol cal (a symbol also used by many for the large calorie). Prefixes are used with this name and symbol, so that the kilogram calorie is often known as the "kilocalorie" and has the symbol kcal.In the medical sciences and non-scientific contexts the calorie is equal to a kilocalorie in the physics or chemistry sense, and is occasionally referred to as a Calorie (capital "C") in an attempt to distinguish it. This has been somewhat ineffective, partly because the convention is not used outside this context, and partly because it results in ambiguity when the word appears at the beginning of a list or sentence. Thus it has to be inferred from the context that the small calorie is not intended.The conversion factor among calories and joules is numerically equivalent to the specific heat capacity of liquid water (in SI units). See "Versions" below for explanation of units.