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 British usage. For this reason, there's some potential for confusion; and some scientists prefer the unit Gya, while others pre… (More on Bya)