How billions and trillions changed
Susie Dent Lexicographer
In 1690, the philosopher John Locke suggested the French word billion as a useful term for avoiding "the often repeating of millions of millions of millions etc". The French had purposely coined "billion" a 100 or so years earlier to denote the second power of a million ("bi" being the standard prefix for two).
However the application of the word was subsequently changed by French arithmeticians, so that the terms "billion" and "trillion" denoted not the second and third powers of a million, but a thousand millions and a thousand thousand millions. This system was adopted by the US. Britain, however, retained the original (and etymologically correct) use.
In 1974, Harold Wilson pledged that the British government would adopt the "short scale" naming system used in the US to avoid ambiguity. As a result, the value of billion is now generally understood to mean a thousand millions. Nonetheless this is still a bone of contention for many, and the older sense "a million millions" is still common.
Full BBC Article - Is trillion the new billion?