1,000,000,000 ("one billion" on the short scale; "one milliard" on the long scale; one thousand million) is the natural number following 999,999,999 and preceding 1,000,000,001. With a number, "billion" can be abbreviated as b, bil or bn.

In standard form, it is written as 1 × 10⁹. The metric prefix giga indicates 1,000,000,000 times the base unit. Its symbol is G.

One billion years may be called an eon in astronomy or 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 not common anymore, and the word has been used to mean one thousand million (1,000,000,000) for several decades.

The term milliard could also be used to refer to 1,000,000,000; whereas "milliard" is rarely used in English, variations on this name often appear in other languages.

In the Indian numbering system, it is known as 100 crore or 1 arab.

1,000,000,000 is also the cube of 1000.

It is a common metric used in macroeconomics when describing national economies.