The Collaborative International Dictionary of English v.0.48:
Quintilllion \Quin*till"lion\, n. [Formed fr. L. quintus the
fifth, after the analogy of million: cf. F. quintillion. See
Quint.]
According to the French notation, which is used on the
Continent and in America, the cube of a million, or a unit
with eighteen ciphers annexed; according to the English
notation, a number produced by involving a million to the
fifth power, or a unit with thirty ciphers annexed. See the
Note under Numeration.
[1913 Webster]