(kcal) A unit of energy defined as the amount of heat required to raise the temperature of one kilogram of pure water by one degree Centigrade under standard conditions (the specific heat of the water at 15 degrees Celsius and the constant pressure of 101.325 kilopascals or one atm being defined as unity), equal to approximately 4.1855 kJ. It is also is used by nutritionists in measuring the energy-producing potential of food as a unit of potential energy contained by a substance, which can be liberated when the material is oxidized, usually by combustion in the presence of oxygen.(NCI)

(calorie) A non-SI unit of energy defined as the amount of heat required to raise the temperature of one kilogram of pure water by one degree Centigrade under standard conditions (the specific heat of the water at 15 degrees Celsius and the constant pressure of 101.325 kPa or one atm being defined as unity), equal to approximately 4.1855 kJ. It is also is used by nutritionists in measuring the energy-producing potential of food as a unit of potential energy contained by a substance, which can be liberated when the material is oxidized, usually by combustion in the presence of oxygen. According to the current national standard in the United States (IEEE/ASTM SI 10 TM-2002. American National Standard for Use of the International System of Units (SI): The Modern Metric System, New York: IEEE, 2002) the calorie in any form is not to be used. Joules should be used instead.

The amount of energy needed to increase the temperature of 1 kilogram of water by 1 degree Celsius. 1 Kilocalorie = 4.185 kJ.