A billion is a numerical value represented by the digit 1 followed by nine zeros, or 1,000,000,000. It is a significant milestone in counting and is often used to describe large quantities, such as population sizes, financial figures, or data storage. In the metric system, a billion is equivalent to one thousand million. In the context of time, a billion seconds is approximately 31.7 years. The term is commonly used in various fields, including economics, science, and technology. For example, billionaire refers to a person whose net worth is at least one billion units of currency.