The Collaborative International Dictionary of English v.0.48:

octane \oc"tane\ ([o^]k"t[=a]n), n. [See Octa-] (Chem.)
   Any one of a group of metametric hydrocarbons (C8H18) of
   the methane (paraffin) series. The most important is a
   colorless, volatile, inflammable liquid, found in petroleum,
   and a constituent of gasoline or ligroin.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

octane
    n 1: any isomeric saturated hydrocarbon found in petroleum and
         used as a fuel and solvent