D defs.my
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. 1.
    Olefiant gas, or ethylene. See Ethylene.(Chem.) [archaic] See: Ethylene
  2. 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.)