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Emission means sending something out.

Thus 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 often refers to the giving off of gases from industrial processes of factories and transportation. As they occur on an industrial scale, even relatively harmless gases can have an undesired effect (such as carbon dioxide being a contributor to the greenhouse effect).

See also:

emission trading

An emission-free piston engine is called an Air Engine.