Why is a billion called a billion?

Answer 1

The prefix bi- means 2. Similarly, the prefix tri- means 3. Originally, the billion was defined as a one followed by 12 zeros - twice as many as the million; while the trillion had three times as many zeros than the million. Somehow, the meaning changed over time, especially in English-speaking countries (to a one with 9 zeros for a billion, and a one with 12 zeros for a trillion).