From 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
   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]
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