Billion - million
There used to be a difference between American and British English in the words used to name the orders of magnitude. Nowadays, this has disappeared. All current writers, on both sides of the Atlantic, use a billion to mean 1 000 000 000 (109) - a thousand million.
- Those who read History, and older texts in any subject, should realise that in Britain before the Second World War, the billion was 1 000 000 000 000 (1012) - a million million. This usage has now been totally abandoned. Similarly, the word milliard (which was used for 1 000 000 000 (109), when a billion was 1 000 000 000 000 (1012)) has ceased to be used in current British English. (It is still current in other European languages.)
For advice on when to use numbers or words, see Numbers or words?.