In thermochemistry, an exothermic reaction is a "reaction for which the overall standard enthalpy change ΔH⚬ is negative." Exothermic reactions usually release heat. The term is often confused with exergonic reaction, which IUPAC defines as "... a reaction for which the overall standard Gibbs energy change ΔG⚬ is negative." A strongly exothermic reaction will usually also be exergonic because ΔH⚬ makes a major contribution to ΔG⚬. Most of the spectacular chemical reactions that are demonstrated in classrooms are exothermic and exergonic. The opposite is an endothermic reaction, which usually takes up heat and is driven by an entropy increase in the system.

वह रासायनिक अभिक्रिया जिसमें उष्मा निकलती है,उष्माक्षेपी अभिक्रिया कहलाती है। इसके विपरीत रासायनिक अभिक्रिया उष्माशोषी कहलाती है। रासायनिक अभिक्रिया के रूप में व्यक्त करने पर -

अभिकारक → उत्पाद + उर्जा