What is Billion?

Legal Definition
A billion is a number with two distinct definitions:

  • 1,000,000,000, i.e. one thousand million, or 109 (ten to the ninth power), as defined on the short scale. This is now generally the meaning in both British and American English.
  • 1,000,000,000,000, i.e. one million million, or 1012 (ten to the twelfth power), as defined on the long scale. This is one thousand times larger than the short scale billion, and equivalent to the short scale trillion.

American English always uses the short scale definition but British English has employed both versions. Historically, the United Kingdom used the long scale billion but since 1974 official UK statistics have used the short scale. Since the 1950s the short scale has been increasingly used in technical writing and journalism, although the long scale definition still enjoys common usage.

Other countries use the word billion (or words cognate to it) to denote either the long scale or short scale billion. For details, see Long and short scales – Current usage.

Another word for one thousand million is milliard, but this is used much less often in English than billion. Some languages such as Spanish, Italian, Dutch, Polish, French, and German, use milliard (or a related word) for the short scale billion, and billion (or a related word) is used for the long scale billion. Thus for the Spanish, Italian, Dutch, Polish, French and German billion is a thousand times larger than the modern English billion.
-- Wikipedia
Legal Definition
A number with nine 0s. It is also shown as a B after a number.