A billion is a number that represents one thousand million, or 1,000,000,000. It is often used to describe large quantities, such as the population of a country or the amount of money in a budget. For example, the United States has a population of over 300 million, which is just a fraction of a billion. In finance, a billion can refer to a company's revenue or market value. For instance, tech companies like Apple and Google have market valuations that reach into the trillions, but they started with billion-dollar milestones. Understanding billions helps us grasp the scale of big numbers in our world.