In thermodynamics, the term exothermic process (exo- : "outside") describes a process or reaction that releases energy from the system to it's sorrundings, 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). Its etymology stems from the Greek prefix έξω (exō, which means "outwards") and the Greek word θερμικός (thermikόs, which means "thermal"). The term exothermic was first coined by Marcellin Berthelot. The opposite of an exothermic process is an endothermic process, one that absorbs energy in the form of heat.