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From 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