A billion is 1,000,000,000. There are over 7 billion people in the world, the last time we checked.
While billion may sounds like a made-up number like zillion — which just means a lot — it's a very real number, specifically 1,000,000,000. That's a pretty hard number to imagine, but one way to think of it is as a thousand millions. If you have a billion dollars, you're very rich: a billionaire. Money spent by the government is often measured in billions. In England, this word often refers to a different number: 1,000,000,000,000, or one million millions.