1. the number that is represented as a one followed by 24 zeros;
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.
WordNet (r) 3.0 (2006):
n 1: the number that is represented as a one followed by 24