The Collaborative International Dictionary of English v.0.48:

Quadrillion \Quad*ril"lion\, n. [F., fr. L. quater four times,
   akin to quattuor four, E. four; -- formed like million. See
   Four, Million.]
   According to the French notation, which is followed also upon
   the Continent and in the United States, a unit with fifteen
   ciphers annexed; according to the English notation, the
   number produced by involving a million to the fourth power,
   or the number represented by a unit with twenty-four ciphers
   annexed. See the Note under Numeration.
   [1913 Webster] Quadrilobate

WordNet (r) 3.0 (2006):

quadrillion
    n 1: the number that is represented as a one followed by 24
         zeros; "in England they call a septillion a quadrillion"
    2: the number that is represented as a one followed by 15 zeros