Exothermic reactions

In thermodynamics, the word exothermic describes a process or reaction that releases energy in the form of heat. Its etymology stems from the Greek prefix ex-, meaning “outside” and the Greek word thermein, meaning “to heat”. The opposite of an exothermic process is an endothermic process, one that absorbs energy in the form of heat. The term “exothermic” was coined by Marcellin Berthelot.
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A chemical or nuclear reaction which gives out energy. The energy comes from the loss of mass according to the Special Theory of Relativity equation E = mc2.