What is Billion ?
Billion is (noun) 1. one thousand million The government raises billions in taxes each year. 2. one million million (dated.) 3. a great many Billions of Christmas cards are sent every year. (NOTE: In American English billion has always meant one thousand million, but in British English it formerly meant one million million, and it is still sometimes used with this meaning. With figures it is usually written bn: $5bn say ‘five billion dollars’.)
source: Easier English, Student Dictionary Upper Intermediate Level