The Collaborative International Dictionary of English v.0.48:

Butylene \Bu"ty*lene\, n. [From Butyl.]
   1. (Chem.) Any one of three metameric hydrocarbons, C4H8,
      of the ethylene series. They are gaseous or easily
      liquefiable; -- called also butene.
      [1913 Webster]

   2. the hypothetical divalent radical -(CH2)4-; -- used in
      combining forms; as, butylene diamine.
      [PJC]