trillion

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The cardinal number equal to 10¹².
• noun Chiefly British The cardinal number equal to 10¹⁸.

from The Century Dictionary.

• noun In the original and most systematic sense, sometimes called English numeration, though of Italian origin, the third power of a million
• noun a million of millions of millions; in the French numeration, usual in the United States, a thousand billions, or a million millions.

from the GNU version of the Collaborative International Dictionary of English.

• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A million million: 1 followed by twelve zeros, 1012.
• noun A million million million: 1 followed by eighteen zeros, 1018.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective one quintillion in Great Britain
• noun the number that is represented as a one followed by 18 zeros
• noun the number that is represented as a one followed by 12 zeros
• noun a very large indefinite number (usually hyperbole)
• adjective one million million in the United States