The Collaborative International Dictionary of English v.0.48:

olefin \o"le*fin\, olefine \o"le*fine\([=o]"l[-e]*f[i^]n or
   [=o]"l[-e]*f[=e]n), n. [From Olefiant.]
   1. (Chem.) Olefiant gas, or ethylene. See Ethylene.
      [archaic]
      [1913 Webster]

   2. Hence: (Chem.) Any one of the series of unsaturated
      hydrocarbons of which ethylene is the type; a member of
      the alkene series of hydrocarbons; an alkene.
      [1913 Webster + PJC]

WordNet (r) 3.0 (2006):

olefin
    n 1: any unsaturated aliphatic hydrocarbon [syn: alkene,
         olefine, olefin]