A chemical or nuclear reaction that liberates heat energy so that heat flows from the Definable part of the universe that can be open, closed, or isolated. An open system exchanges both matter and energy with its surroundings. A closed system can only exchange energy with its surroundings; it has walls through which heat can pass. An isolated system cannot exchange energy or matter with 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.