An exothermic reaction is a chemical reaction where the substances reacting release energy as heat. An example of this is combustion. Exothermic reactions transfer energy to the surroundings. The reaction that does the opposite (it absorbs heat) is an endothermic reaction.
The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to become hotter. The temperature increase can be detected using a thermometer, and the thermal energy by a calorimeter. Some examples of exothermic reactions are: