Temperature and Heat

The Kinetic Molecular theory states that as a substance is heated, molecules gain energy. However, we cannot measure the amount of energy in the form of heat a substance contains, but we can measure the flow of heat energy. This is always from a warm to cold body until the temperature equalizes. This change is used to measure heat flow.

Exothermic: If there's an output of energy out of the system, then the system is losing energy.

Endothermic: If there's an input of energy into the system, then the system is gaining energy.

Endothermic vs. Exothermic

Source