Noun
(automotive, aviation) The process of squirting a small amount of water, in the form of a fine spray, into the air/fuel mixture going into a gasoline piston engine, in order to cool the engine cylinders, and, thereby, reduce detonation or knocking in high performance engines, especially when using a high compression ratio.
(aviation) The process of squirting water into the combustion chamber of a jet engine to allow it to produce greater thrust, partly from the production of high-pressure steam and partly from the cooling effect of the water allowing the engine to burn a greater quantity of fuel without overheating.
Source: en.wiktionary.org