The Collaborative International Dictionary of English v.0.48:

butene \butene\ n.
   any of three isomeric singly unsaturated hydrocarbons C4H8;
   all are used in making synthetic rubbers; -- called also
   butylene.
   [WordNet 1.5]

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]

WordNet (r) 3.0 (2006):

butene
    n 1: any of three isomeric hydrocarbons C4H8; all used in making
         synthetic rubbers [syn: butylene, butene]