Heat
Heat refers to Energy in the process of being transferred across the boundary between a Thermodynamic system and its environment due to a difference in Temperature. For heat transfer to take place, two or more objects must be in Thermal contact.
The flow of heat into a system is given the symbol
Heat is closely related to temperature, but the two are not the same. See The relationship between temperature and heat in a solid or liquid. Adding heat can also result in a phase change — see Heat of transformation.