billion

xyu201

\"Billion\" is a word which is somewhat ambiguous in U.K. practice, but
not in American. For several centuries in Britain, it meant \"a million
million\" (1,000,000,000,000 = 10^12 = US trillion), but in 1974, the
British government announced that it would use \"billion\" to mean 10^9,
the same as US usage. For the last several decades, English-language
publishers have used \"billion\" or \"thousand million\" for 1,000,000,000 =
10^9 = US billion.

The \"old\" use is still encountered just often enough (in speech,
informal writing, and older books) to cause some Britons to be unsure of
what the speaker or writer means by the word.

The first few U.S. words for large numbers, and the corresponding
traditional British terms, are as follows:

     U.S.         Traditional British
10^6   million       million
10^9   billion       thousand million or milliard
10^12  trillion       billion or million million
10^15  quadrillion     thousand billion
10^18  quintillion     trillion

Scientists have long preferred to express numbers in figures rather than
in words, so it is easy to avoid \"billion\" in contexts where precision
is required. The plural, \"billions\", is still used freely with the
colloquial meaning of \"a very large number\".

Some articles with more history about these terms:
http://www.unc.edu/~rowlett/units/large.html