Eli Bendersky's website A curious fact: how much "billion" is depends on where you live, and what language you speak.
In the American system, a billion is 1,000 millions, that is 10e9. In the British system, it's 10e12 (million millions). 10e12 is "trillion" in the American system - but hold on, it's 10e18 in the British system. 10e18 in American is "quintillion" but 10e30 in British.
Now, in Spanish a billón follows the British system: 10e12. An "American" billion in Spanish is "mil millones" - literally "a thousand millions".
To make things more interesting, there's another name for 10e9 - the "milliard". It's only rarely used in English, but is routinely used in Russian and Hebrew to describe 10e9. In fact, neither in Russian nor in Hebrew the word "billion" is used at all. "Trillion" in Russian and Hebrew follows the American system - 10e12