/ ˈbɪljən; NAmE / number ( plural verb )
1.
( abbr. bn ) 1 000 000 000; one thousand million :
Worldwide sales reached 2.5 billion.
half a billion dollars
They have spent billions on the problem (= billions of dollars, etc.) .
HELP NOTE : You say a, one, two, several, etc. billion without a final 's' on 'billion'. Billions (of ... ) can be used if there is no number or quantity before it. Always use a plural verb with billion or billions , except when an amount of money is mentioned
Two billion (people) worldwide are expected to watch the game.
Two billion (dollars) was withdrawn from the account.
There are more examples of how to use numbers at the entry for hundred .
2.
a billion or billions ( informal ) a very large amount :
Our immune systems are killing billions of germs right now.
3.
( old-fashioned , BrE ) 1 000 000 000 000; one million million
SYN trillion
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WORD ORIGIN
late 17th cent.: from French , from million , by substitution of the prefix bi- two for the initial letters.