1,000,000,000 (one billion, short scale; one thousand million or milliard, yard, long scale) is the natural number following 999,999,999 and preceding 1,000,000,001. One billion can also be written as b or bn.
One billion years may be called eon (or aeon) in astronomy and geology.
Previously in British English (but not in American English), the word "billion" referred exclusively to a million millions (1,000,000,000,000). However, this is no longer as common as earlier, and the word has been used to mean one thousand million (1,000,000,000) for some time. The alternative term "one thousand million" is mainly used in the U.K., or countries such as Spain that uses "one thousand million" as one million million constitutes a billion. The worded figure, as opposed to the numerical figure (one thousand million/1,000,000,000) is used to differentiate between "one thousand million" or "one billion".
The term milliard can also be used to refer to 1,000,000,000; whereas "milliard" is seldom used in English, variations on this name often appear in other languages.