Exothermic
Definition
A process that releases heat to its surroundings, like burning wood or a hand warmer activating. The enthalpy change is negative.
Technical definition
An exothermic process releases heat to the surroundings at constant pressure (deltaH negative), raising the surroundings' temperature. Examples include combustion, many bond-forming reactions, and condensation.
Example
Burning natural gas in a furnace is exothermic: heat flows from the flame into your house.
Common confusion
Exothermic describes heat flow, not spontaneity. Some exothermic processes are nonspontaneous, and some endothermic processes, like ice melting at room temperature, happen spontaneously.