Heat flux also named as thermal flux, is referred to as heat flux density, heat-flow density is a flow of energy per unit of area per unit of time. In SI its units are watts per square meter. As heat flux has both a direction and a magnitude, and so it is a vector quantity.

Heat flux by the convection process is directly proportional to the temperature difference between solid, liquid, or gaseous media participating in heat transfer. Under the conduction process, the heat flux vector is directly proportional to and usually parallel to the temperature gradient vector. The heat flux formation due to radiation is a flux of electromagnetic radiation. In contrast to convection and heat conduction, it may occur without any intervening medium.