methane
    n 1: a colorless odorless gas used as a fuel

Methane \Meth"ane\, n. [See Methal.] (Chem.)
   A light, colorless, gaseous, inflammable hydrocarbon, CH4;
   marsh gas. It is the simplest of the aliphatic hydrocarbons.
   See Marsh gas, under Gas.
   [1913 Webster]

   Methane series (Chem.), a series of saturated hydrocarbons,
      of which methane is the first member and type, and
      (because of their general chemical inertness and
      indifference) called also the paraffin (little affinity)
      series. The lightest members are gases, as methane,
      ethane; intermediate members are liquids, as hexane,
      heptane, etc. (found in benzine, kerosene, etc.); while
      the highest members are white, waxy, or fatty solids, as
      paraffin proper.
      [1913 Webster]

methanogen \methanogen\ n.
   A type of archaebacteria found in anaerobic environments such
   as animal intestinal tracts or sediments or sewage, and
   capable of producing methane; a source of natural gas.
   [WordNet 1.5]