Entry 2 senses Webster, 1913 olefin /(ō"lē̇*fĭn [or] ō"lē̇*fēn)/ · o·le·fin · IPA /ˈəʊləfɪn/ 01 n. Olefiant gas, or ethylene. See Ethylene. 1. Olefiant gas, or ethylene. See Ethylene.(Chem.) [archaic] See: Ethylene 2. Any one of the series of unsaturated hydrocarbons of which ethylene is the type; a member of the alkene series of hydrocarbons; an alkene.(Chem.)