In English, a “billion” is 1 000 000 000 (a thousand million).
This has always been the case in US English.
In British English, in the past the word “billion” meant a million million. If we wanted to refer to a thousand million, we simply said “thousand million” or more rarely “milliard”. But in 1974 we officially adopted the US practice of using “billion” to mean a thousand million.
The word “milliard” has since gone out of use in British English. It never existed in US English.
Much of the confusion over the usage of these words derives from variants of the word “milliard” remaining in common usage (and meaning a thousand million) in other European languages, e.g. Spanish millardo, French milliard, German milliarde, Polish miliard and Russian миллиард.
Remember these words translate into “billion”. And there is no longer any distinction between British and US usage.
For more information see http://en.wikipedia.org/wiki/Long_and_short_scales
Common Mistakes in Business English 
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Cool! I was reading an older book and they used the term milliard, which I thought might be another way of saying million. I’m glad I looked it up; this was interesting.
Billion is an abbreviation of bi-million or million to the power of two. Milliard, or better still Thillion, should be used for thousand million. Would then allow the words to match the numbers. Bi-million, Tri-million, Quiad-illion, etc
There’s a good explanation of the difference between the long and short scales and the logic behind the names of the numbers here: https://en.wikipedia.org/wiki/Long_and_short_scales
I’m not a native speaker of English, but I can imagine why the usage of the word ‘milliard’ has been dropped. There are 2 reasons that I think of.
1. The first part of words like million, billion, trillion, etc. are meant to tell us how many groups of 3 digits there are, ignoring the digits that have no comma/dot coming before them as well as the first group of three 0s (in case of round numbers how many groups of 0s). It’s an easier way to write down or read the whole number. Think of bi-, tri-, quad-, quint-, which just means that there are 2, 3, 4 or 5 groups of three 0s after the 1 and the first three 0s respectively. All groups of 3 digits will be separated by commas in US English and by dots in UK English. The confusing part of the English wording is that the number of groups of 0s is always one more than the first part of the word is meant to represent. I think that’s why other European languages stick with the ‘milliard’-like thing.
2. In a lot of other European languages, including my native language, Dutch, the term ‘miljard’ literally means ‘milliard’, while the number is now officially translated in English as ‘billion’. The logic behind the system Dutch uses is based on two groups of three 0s (with a total of six 0s) in addition to that of a single group of three 0s. But in addition to that, the first group of 0s in the number is not ignored. For example: ‘ 1 triljoen’ is the term for a number which has a 1 with 3 pairs of groups of 0s in it (1,000,000,000,000,000,000/1.000.000.000.000.000.000). In English, this is 1 quintillion. Since in Dutch 1 million means ‘1 miljoen’ (1 group pair of 0s) and 1 trillion is ‘1 biljoen’ (2 group pairs of 0s), something has to come in between. That’s why we still use ‘-ard’ instead of ‘-oen’ to represent a number with group pairs that also has a single group in addition to it. For example: ‘1 biljard’ is 1 quadrillion, or 1.000.000.000.000.000 (two group pairs + one single group of 0s). And that is probably where the lingual problem comes in, because I suspect that they don’t want English speakers to get confused by the word ‘bilyards’ which is something else than a number.
Thanks William – very intereresting. But groups of 3 digits are separated by commas in both UK and US English. We use dots for decimals.
This is an issue that has vexed me for some time. As mentioned above, bi, tri, quad etc. meaning 2,3,4, … these being powers of million. Billiard and Milliard are old words for thousand million which did not please the American thousand millionaires. Rather than calling themselves billionaires they should be more accurately described as thillionaires where thillion is a thousand million. Any thoughts?
In all of Europe they always and still use the correct and true system of the Milliarde which we in the United States see as a Billion. Our Trillion is their Billion.
The numbering system for millions is rather confusing. If a million to the power 2 is a tri-million then a bike with 2 wheels must be a tricycle! How about using THILLION for thousand million and use billion to mean million million. We could then have thoubillion, trillion, thoutrillion, quadillion, etc.