The calorie is a unit measure of the quantity of heat; 1cal=4.1868 Joule (J).
Before the universal adoption of the SI systems, the calorie was included in the dimensions of some functions which characterize thermodynamic and translational properties of substances and heat transfer processes. These functions are: internal energy, enthalpy, the Gibbs energy (isobaric-isothermal potential), the Helmholtz energy (isochoric-isothermal potential), the partial molar Gibbs energy (chemical potential), entropy, heat capacity, heat conduction and heat transfer coefficients.
Historically, the calorie has been defined as the quantity of heat required for heating up 1 g of distilled water at atmospheric pressure by 1°C. The reason for the discrepancy in the value of calorie— which existed in literature (in particular, the so-called thermochemical calorie, 1cal=4.1840 J)—is due to the temperature dependence of the heat capacity of water. At present, the Joule, which represents the energy measurement unit in the SI system, is most frequently used in measurements and tabulations of the functions mentioned above.