Definition of "Billion" :

noun: the number that is represented as a one followed by 9 zeros

noun: the number that is represented as a one followed by 12 zeros; in the United Kingdom the usage followed in the United States is frequently seen

noun: a very large indefinite number (usually hyperbole)

adjective: denoting a quantity consisting of one thousand million items or units in the United States

adjective: denoting a quantity consisting of one million million items or units in Great Britain