(blank) 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, specifically thermal energy.

An important characteristic of heat is that it is an energy in transit, so it flows from one system to another when there is a difference in temperature  between them.

Heat will always flow from the system with a higher temperature to the system with a lower temperature, never the other way around.

Since heat is an energy in transit, it cannot be stored in a system, it can only be absorbed or transferred.

e/c, where e is its total energy