French billion a million millions, from bi-¹ + -illion as in million. Large numbers used to be written in groups of six figures; one billion was 1 plus two groups of six noughts (1 000000 000000). In 17th-cent. France it became customary to use groups of three figures; the idea of a million squared was lost and a billion came to be written 1 000 000 000 – a thousand million. This sense was adopted into American English and, to avoid confusion, is now generally used in British English