A chemical reaction that releases energy in the form of heat is called an exothermic reaction.
In thermochemistry, an exothermic reaction has a negative enthalpy change, written as ΔH < 0. The products contain less chemical energy than the reactants, and the difference is transferred to the surroundings, often raising their temperature. Burning fuels, acid–base neutralization, thermite reactions, and the oxidation inside disposable hand warmers are familiar examples.
The word is built from Greek roots meaning “outside” and “heat,” reflecting heat moving out of the reacting system. A reaction can also release light, sound, or other energy, but “exothermic” specifically emphasizes heat transfer. The opposite is endothermic, in which heat is absorbed. Exothermic should not be confused with exergonic: exergonic describes a negative Gibbs free-energy change, a related but different thermodynamic idea.