What is "cnh2n"

Ethylene \Eth"yl*ene\ ([e^]th"[i^]l*[=e]n), n. [From Ethyl.] (Chem.) A colorless, gaseous hydrocarbon, C2H4, forming an important ingredient of illuminating gas, and also obtained by the action of concentrated sulphuric acid in alcohol. It is an unsaturated compound and combines directly with chlorine and bromine to form oily liquids (Dutch liquid), -- hence called olefiant gas. Called also ethene, elayl, and formerly, bicarbureted hydrogen.

Ethylene series (Chem.), the series of unsaturated hydrocarbons of which ethylene is the type, and represented by the general formula CnH2n.