A billion is a number equal to one thousand million, or 1,000,000,000. That's a one followed by nine zeros. To understand how huge this is, imagine counting to a billion out loud. If you counted one number per second without stopping, it would take you about 32 years to reach a billion.

A billion is so large that it's hard to picture in everyday life. If you stacked a billion pennies, the tower would reach about 870 miles high, far beyond where airplanes fly. If you had a billion dollars and spent $1,000 every single day, it would take you about 2,740 years to run out of money.

The word billion becomes especially important when discussing very large things: the number of stars in a galaxy, the age of Earth (about 4.5 billion years), government budgets, or the wealth of major companies. You'll also hear about billionaires, people who have a billion dollars or more. These are among the wealthiest people in the world.