The Collaborative International Dictionary of English v.0.48:

Septillion \Sep*til"lion\, n. [F. septilion, formed fr. L.
   septem seven, in imitation of million.]
   According to the French method of numeration (which is
   followed also in the United States), the number expressed by
   a unit with twenty-four ciphers annexed. According to the
   English method, the number expressed by a unit with forty-two
   ciphers annexed. See Numeration.
   [1913 Webster]

WordNet (r) 3.0 (2006):

septillion
    n 1: the number that is represented as a one followed by 24
         zeros