1. In nutrition, a unit of measure of food energy (symbol: C) equivalent to 1 kilocalorie (kcal) or 4184 joules. (Use of kcal is more common outside North America)
  2. In chemistry, a calorie (symbol: cal) is the amount of energy needed to raise the temperature of one gram of water by one degree Celsius at a pressure of one atmosphere Approximately 4.2 joules. It has largely been superseded by the the joule.

The ambiguity surrounding the various uses of the term is a holdover from when it was coined in France in the 19th century at which time there was no metric standard for energy or heat.

In 1948 the Consultative Committee on Thermometry recommended discarding the calorie, because it could not be derived directly from basic units. In 1954, the Système International (SI) officially adapted the joule and in 1970, the Committee on Nomenclature of the American Institute of Nutrition advised that the kilocalorie should be replaced by the kilojoule (kJ) in scientific publications. Nonetheless, both the Calorie and the kilocalorie both persist today as a unit of food energy.

See for complete history.

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