Source : Webster's Revised Unabridged Dictionary (1913)

Billion \Bil"lion\, n. [F. billion, arbitrarily formed fr. L.
   bis twice, in imitation of million a million. See {Million}.]
   According to the French and American method of numeration, a
   thousand millions, or 1,000,000,000; according to the English
   method, a million millions, or 1,000,000,000,000. See
   {Numeration}.

Source : WordNet®

billion
     n 1: the number that is represented as a one followed by 12
          zeros; in the United Kingdom the usage followed in the
          United States is frequently seen [syn: {one million
          million}, {1000000000000}]
     2: the number that is represented as a one followed by 9 zeros
        [syn: {one thousand million}, {1000000000}]

billion
     adj 1: denoting a quantity consisting of one thousand million items
            or units in the United States [syn: {a billion}]
     2: denoting a quantity consisting of one million million items
        or units in Great Britain [syn: {a billion}]