Answer:
Heat
Step-by-step 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.