In astronomy, geology, and paleontology, bya or "b.y.a." is an acronym for billion years ago. This abbreviation is commonly used as a unit of time to denote length of time before the present. Specifically, one bya is equal to 109 years ago.
The "billion" in bya is, by convention, the short-scale billion (109) of American preference, not the long-scale billion (1012) of European usage. For this reason, there is some potential for confusion; and some scientists prefer the unit Gya, while others prefer Ga (Giga annum). However, bya remains in more widespread use.
Like the related unit mya ("million years ago"), bya is traditionally written in lowercase. Ga or Gya has a capitalized first letter instead.