Heat is a type of energy transfer that is caused by a temperature difference, and it can change the temperature of an object. Since heat is a form of energy, its SI unit is the joule (J). Another common unit of energy often used for heat is the calorie (cal), which is defined as the energy needed to change the temperature of 1 g of water by 1 °C, specifically between 14.5 °C and 15.5 °C, since the energy needed shows a slight temperature dependence. Another commonly used unit is the kilocalorie (kcal), which is the energy needed to change the temperature of 1 kg of water by 1 °C. Since mass is most often specified in kilograms, the kilocalorie is more convenient. Confusingly, food calories are actually kilocalories, a fact not easily determined from package labeling.

Heat transfer is the movement of energy from one place or material to another as a result of a difference in temperature. Heat transfer is fundamental to everyday activities such as home heating and cooking, as well as many industrial processes.

Specific heat depends on the material and its phase. For example, the specific heat of water is 5 times that of glass and 10 times that of iron, which means that it takes 5times as much heat to raise the temperature of the water compared to glass and 10 times as much compared to iron. In fact, water has one of the largest specific heats of any material, which is important for sustaining life on Earth. The value of specific heat must generally be measured because there is no simple way to calculate it precisely.