In astronomy, geology, and paleontology, bya is an acronym for 'billion years ago'. This abbreviation is commonly used to show the length of time before the present or "B.P." (before AD 1950). Specifically, one bya is equal to 109 years ago.
Bya is traditionally written in lower case (small letters). It is often replaced in scientific literature by a term based on Latin: Ga (gigaannum).