Definition of billion number from the Oxford Advanced Learner's Dictionary

      

    billion

     number
    number
    BrE BrE//ˈbɪljən//
     
    ; NAmE NAmE//ˈbɪljən//
     
    (plural verb)
     
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  1. 1  (abbreviation 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.). 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an amount of money is mentionedTwo billion (people) worldwide are expected to watch the game.Two billion (dollars) was withdrawn from the account.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. 2  a billion, billions (of…) (informal) a very large amount Our immune systems are killing billions of germs right now.
  3. 3(old-fashioned, British English) 1 000 000 000 000; one million million synonym trillion
  4. Word Origin late 17th cent.: from French, from million, by substitution of the prefix bi- ‘two’ for the initial letters.
See the Oxford Advanced American Dictionary entry: billion