billion

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

• noun The cardinal number equal to 10⁹.
• noun Chiefly British The cardinal number equal to 10¹².
• noun An indefinitely large number.

from The Century Dictionary.

• noun In Great Britain, a million of millions; as many millions as there are units in a million (1,000,000,000,000).
• noun In France and the United States, a thousand millions (1,000,000,000).

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

• noun According to the French and American method of numeration, a thousand millions, or 1,000,000,000; according to the English method, a million millions, or 1,000,000,000,000. See numeration.

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

• noun US A milliard, a thousand million: 1 followed by nine zeros, 109.
• noun obsolete A million million: a 1 followed by twelve zeros, 1012.
• noun colloquial, plural, hyperbolic A very large number.

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

• noun the number that is represented as a one followed by 9 zeros
• adjective denoting a quantity consisting of one million million items or units in Great Britain
• noun a very large indefinite number (usually hyperbole)
• adjective denoting a quantity consisting of one thousand million items or units in the United States
• noun the number that is represented as a one followed by 12 zeros; in the United Kingdom the usage followed in the United States is frequently seen