Formerly in the UK, one million million, 1012, but the US meaning of one thousand million, 109, has now been adopted worldwide.
Mathematics
A thousand million (109). This is now standard usage, though in Britain billion used to mean a million million (1012).
Chemical Engineering
One thousand million (109). In the UK this was formerly one million million (1012) and was changed when the British Treasury started using the term billion in the American sense in the 1960s.
Economics
One thousand million (= 109) in the US and modern British system. A billion was a million million (= 1012) in the traditional British system but this is no longer in use.