Definitions
from The American Heritage® Dictionary of the English Language, 4th Edition
 n. The cardinal number equal to 1012.
 n. Chiefly British The cardinal number equal to 1018.
from Wiktionary, Creative Commons Attribution/ShareAlike License
 n. A million million: 1 followed by twelve zeros, 1012.
 n. A million million million: 1 followed by eighteen zeros, 1018.
from the GNU version of the Collaborative International Dictionary of English
 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.
from The Century Dictionary and Cyclopedia
 n. In the original and most systematic sense, sometimes called English numeration, though of Italian origin, the third power of a million
 n. a million of millions of millions; in the French numeration, usual in the United States, a thousand billions, or a million millions.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
 adj. one quintillion in Great Britain
 n. the number that is represented as a one followed by 18 zeros
 n. the number that is represented as a one followed by 12 zeros
 n. a very large indefinite number (usually hyperbole)
 adj. one million million in the United States
Etymologies
from The American Heritage® Dictionary of the English Language, 4th Edition
from Wiktionary, Creative Commons Attribution/ShareAlike License
Examples

If the risk of sea level rise, species extinctions, disrupted agriculture, new infrastructure investments required to adapt to catastrophic climate change, etc can be valued at $20 trillion in "net present value" (NPV ... value today, in today's dollars), then an investment of $trillion or $4 trillion to obviate that risk is a wellspent insurance policy.

The term trillion makes all of this somewhat hard to grasp.

I would have not known this, but I quickly learned it by looking up the term trillion in WolframAlpha.
Dr. Peter Jacso Reviews Wolfram Alpha in His Final Gale.com Review « ResourceShelf

I've read that mathematicians do not even use the term trillion!
Polls Put Obama Ahead Nationally, Race Is Close In Key States

At that point, the word trillion was rarely mentioned in the news and resided more in the imagination of children, alongside zillions and gazillions, as an expression of something on an enormous scale.

We all went to school and we learned a trillion is a million million or a thousand billion.

You don't know what a trillion is and neither did 1.

Once a representation of something beyond our comprehension, the word trillion  1,000,000,000,000  has now sealed its place in common parlance.

The first time someone hears the word trillion, they might not know the number of zeros but they know it's a big number.

Now that the word "trillion" is used in everyday conversation regarding Obama, it is sooooo much easier to drop a few billion here and there without anyone questioning it.
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