Calories

The name calorie is used for two units of energy:

• the small calorie or gram calorie (symbol: cal) which is the approximate amount of energy needed to raise the temperature of one gram of water by one degree Celsius;
  
• large calorie, kilocalorie, kilogram calorie, dietary calorie, nutritionist's calorie or food calorie (symbol: kcal or Cal), equal to 1000 small calories.

These units were at one time part of the metric system, but now they have been superseded in the International System of Units by the joule. One small calorie is about 4.2 joules, and one large calorie is therefore 4.2 kilojoules. The factors used to convert calories to joules are numerically equivalent to expressions of the specific heat capacity of water in joules per gram or per kilogram. The conversion factor depends on the definition adopted.
In spite of its non-official status, the (large) calorie is still widely used as a unit of food energy in the US, UK and some other Western countries. The small calorie is also often used in chemistry.
The calorie was first defined by Nicolas Clément in 1824 as a unit of heat, and entered French and English dictionaries between 1841 and 1867. The word comes from Latin calor meaning "heat".