The Collaborative International Dictionary of English v.0.48:

Octillion \Oc*til"lion\, n. [L. octo eight + -illion, as in E.
   million: cf. F. octillion.]
   According to the French method of numeration (which method is
   followed also in the United States) the number expressed by a
   unit with twenty-seven ciphers annexed. According to the
   English method, the number expressed by a unit with
   forty-eight ciphers annexed. See Numeration.
   [1913 Webster] Octo

WordNet (r) 3.0 (2006):

octillion
    n 1: the number that is represented as a one followed by 27
         zeros