Ah, I could bore you all for hours on this one but I'll keep it brief.
The English definition of a billion was a million-million.
The American definition was a thousand-million, what we Brits would have referred to as a millard.
Globalisation of financial and other markets forced standardisation and the American system won, despite lacking the clear logical naming structure of the British system (billion had twice as many zeros as a million, trillion had three times as many, quarillion four, etc).
In short, the "business billion", i.e. the number we should use in calculations etc is a thousand-million.
A final point though, particularly in the UK, is to be aware that due to the age of many pensioners and policyholders, they may believe we are referring to the old english billion if any communications are denominated such. We should therefore take care to clarify any use of the term to ensure our communications are appropriate for the reader.
This site has both American and British names for 10^x multiples - to reiterate we use the American system in finance work.
http://www.uni-bonn.de/~manfear/numbers_names.php
