billion (Adjective) — Denoting a quantity consisting of one thousand million items or units.
billion (Adjective) — Denoting a quantity consisting of one million million items or units.
billion (Noun) — The number that is represented as a one followed by 9 zeros.
billion (Noun) — The number that is represented as a one followed by 12 zeros; in the United Kingdom the usage followed is frequently seen.
billion (Noun) — A very large indefinite number (usually hyperbole). ex. "there were billions of flies"