trillion

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The cardinal number equal to 10¹².
2. n. Chiefly British The cardinal number equal to 10¹⁸.

Century Dictionary and Cyclopedia

1. n. In the original and most systematic sense, sometimes called English numeration, though of Italian origin, the third power of a million
2. n. a million of millions of millions; in the French numeration, usual in the United States, a thousand billions, or a million millions. In Italian arithmetics from the last quarter of the fifteenth century the words bilione or duilione, trilione, quadrilione or quattrilione, quintilione, cinquilione, or quinquilione, seilione or sestilione, settilione, ottilione, novilione, and decilione occur as common abbreviations of due volte millioni, tre volte millioni, etc. In other countries these words came into use much later, although one French writer, Nicolas Chuquet, mentions them as early as 1484, in a book not printed until 1881. The Italians had, besides, another system of numeration, proceeding by powers of a thousand. The French, who, like other northern peoples, took most if not all their knowledge of modern or Arabic arithmetic from the Italians, early confounded the two systems of Italian numeration, counting in powers of a thousand, but adopting the names which properly belong to powers of a million. The result has been that the names billion, trillion, etc., have, owing to their ambiguity, been almost discarded. A trilliar, or a thousand millions, is called a milliard by bankers, and when a name for a thousand milliards comes to be wanted it is probable that some other augmentative form will be borrowed from the Italian or Spanish. Compare billion.

Wiktionary

1. n. A million million: 1 followed by twelve zeros, 1012.
2. n. A million million million: 1 followed by eighteen zeros, 1018.

GNU Webster's 1913

1. n. 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.

WordNet 3.0

1. adj. one quintillion in Great Britain
2. n. the number that is represented as a one followed by 18 zeros
3. n. the number that is represented as a one followed by 12 zeros
4. n. a very large indefinite number (usually hyperbole)
5. adj. one million million in the United States