Trillion \Tril"lion\, n. [F. trillion, formed from the pref. tri- in imitation of million a million. Cf. Billion.]

According to the French notation, which is used upon the Continent generally and in the United States, the number expressed by a unit with twelve ciphers annexed; a million millions; according to the English notation, the number produced by involving a million to the third power, or the number represented by a unit with eighteen ciphers annexed. See the Note under Numeration.

Source: Webster's Revised Unabridged Dictionary (1913)