- The definition of exothermic is a process or chemical reaction characterized by or causing the liberation or release of heat.
Combustion where heat is released is an example of an exothermic reaction.
Origin of exothermicexo- + thermic
(comparative more exothermic, superlative most exothermic)
From French exothermique, from exo- "outside" + thermique "of heat", both ultimately from Ancient Greek.