The Collaborative International Dictionary of English v.0.48:

Propylene \Pro"pyl*ene\, n. [Cf. F. propyl[`e]ne.] (Chem.)
   A colorless gaseous hydrocarbon (C3H6) of the ethylene
   series, having a garlic odor. It occurs in coal gas, and is
   produced artificially in various ways. Called also propene.
   [1913 Webster]

WordNet (r) 3.0 (2006):

propylene
    n 1: a flammable gas obtained by cracking petroleum; used in
         organic synthesis [syn: propylene, propene]