Source : Webster's Revised Unabridged Dictionary (1913)

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}.