[ noun ] any unsaturated aliphatic hydrocarbon <noun.substance>
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]
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]