Heat capacity is the ratio of heat absorbed by the substance corresponding to its temperature change. It is also called thermal capacity and the SI unit of heat capacity is J/K. Whereas, specific heat capacity is defined as the amount of heat necessary to change the temperature of 1 kg of a substance by 1 K and is also called massic heat capacity. Its SI unit is J/kg⋅K.

Molar heat capacity quantifies the ratio of the amount of heat added (or removed) to increase (or decrease) the temperature of 1 mole of a substance by 1 K, either measured under a constant volume or constant pressure. Molar heat capacity is one of the characteristics of a substance and is also called molar specific heat. Its SI unit is J/mol·K. Measuring the molar heat capacity at constant volume is the easiest. The system acquires infinite number of possible heat capacities if neither the pressure nor the volume is constant. In most cases, the molar heat capacity at constant pressure (C_P) is higher than the molar heat capacity at constant volume (C_V). For example, the air has 40% greater C_P than C_V.