In astronomy, geology, and paleontology, bya or "b.y.a." is an initialism 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 109 "billion" of the short scale of the U.S., not the long-scale 1012 "billion" of some European usage. For this reason, there is potential for some confusion; and some scientists prefer the unit Gya, while others prefer Ga (Giga annum). However, bya remains in more widespread use. In 1974 the U.K. switched from the long scale to the short scale.