The Collaborative International Dictionary of English v.0.48:

Milliard \Mil`liard"\ (F. m[-e]l`y[.a]r"; E.
   m[i^]l"l[i^]*[aum]rd), n. [F., from mille, mil, thousand, L.
   mille.]
   A thousand millions; -- usually called billion in the
   United States. See Billion.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

billion
    adj 1: denoting a quantity consisting of one thousand million
           items or units in the United States
    2: denoting a quantity consisting of one million million items
       or units in Great Britain
    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: billion, one
         million million, 1000000000000]
    2: a very large indefinite number (usually hyperbole); "there
       were millions of flies" [syn: million, billion,
       trillion, zillion, jillion, gazillion]
    3: the number that is represented as a one followed by 9 zeros
       [syn: billion, one thousand million, 1000000000]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "billion":
   a billion, a crore, a lakh, a million, a myriad, a nonillion,
   a quadrillion, a thousand, a zillion, astronomical number, googol,
   googolplex, infinitude, infinity, jillion, large number, milliard,
   trillion, zillion