Source : Webster's Revised Unabridged Dictionary (1913)

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}.

Source : WordNet®

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