is the thermal energy that flows from one substance to another when the substances differ in temperature.

Heat

Explanation:

Heat is a form of energy (thermal energy) in transition, which is transferred by non-mechanical means due to a temperature difference.

Heat always flows from the higher temperature system to the lower temperature system.

Its nature is transitory, so it makes no sense to speak of heat stored in a system, but of heat flowing from one system or substance to another given a temperature difference.

If there is no difference in temperature, no heat will flow between systems or substances.