The Collaborative International Dictionary of English v.0.48:

Sextillion \Sex*til"lion\, n. [Formed (in imitation of million)
   fr. L. sextus sixth, sex six: cf. F. sextilion.]
   According to the method of numeration (which is followed also
   in the United States), the number expressed by a unit with
   twenty-one ciphers annexed. According to the English method,
   a million raised to the sixth power, or the number expressed
   by a unit with thirty-six ciphers annexed. See Numeration.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sextillion
    n 1: the number that is represented as a one followed by 21
         zeros