The word emission generally means sending something out. It can be used in the following contexts:

• In chemistry emissions are the products of a reaction, either chemical or nuclear.
• In physics and physical chemistry emissions are outputs of electromagnetic radiation or particles.
• In common usage, emission is often the giving off of gases from industrial processes of factories and transport. As they occur on an industrial scale, even relatively harmless gases can have an undesired effect (such as carbon dioxide contributing to the greenhouse effect). See also Emission trading, automobile emissions control,greenhouse gas and pollution.
• In the history of optics, light was supposed by adherents of emission theory to be emitted by the eyes. Visual perception was accomplished by such rays of light acting like feeling hands.