A chemical or nuclear reaction that liberates heat energy so that heat flows from the system to the surroundings. Exothermic reactions cause an increase in temperature.

A classic exothermic reaction is the mixing of a strong acid with water. In order to prevent an explosive and dangerous reaction, the acid should be slowly added to the water thus creating a very dilute solution releasing a small amount of heat over time instead of immediately vaporizing the solution if all the acid and water are dumped together.