In thermodynamics, the term exothermic process (exo- : outside) describes a process or reaction that releases energy from the system, usually in the form of heat, but also in a form of light (e.g. a spark, flame, or flash), electricity (e.g. a battery), or sound (e.g. explosion heard when burning hydrogen).xothermic (and endothermic) describe two types of chemical reactions or systems found in nature, as follows. Simply stated, after an exothermic reaction, more energy has been released to the surroundings than was absorbed to initiate and maintain the reaction.