number,the number equivalent to the product of a thousand and a million; 1,000,000,000 or 109. 1,000,000,000 is the natural number following 999,999,999 and preceding 1,000,000,001. A billion is a number with two distinct definitions: 1,000,000,000, i. In British English, a billion used to be equivalent to a million million (i. The old UK meaning of a billion was a million million, or one followed by twelve noughts (1,000,000,000,000). We discuss millions, billions, trillions and centillions is this film about the long and short scales.