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

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "octane":
   alcohol, benzine, briquette, burnable, butane, carbon, charcoal,
   coal, coke, combustible, dope, ethane, ethanol, fireball, firing,
   flammable, flammable material, fuel, fuel additive, fuel dope, gas,
   gas carbon, gasoline, heptane, hexane, inflammable,
   inflammable material, isooctane, jet fuel, kerosene, methane,
   methanol, natural gas, oil, paraffin, peat, pentane, propane,
   propellant, rocket fuel, turf