bya or b.y.a. is an abbreviation for "billion years ago". It is commonly used as a unit of time to denote length of time before the present in 109 years. This initialism is often used in the sciences of astronomy, geology, and paleontology.
The "billion" in bya is the 109 "billion" of the short scale of the U.S., not the long-scale 1012 "billion" of some European usage. Billion by this convention (109) is often called a "thousand million" in the UK and a "milliard" in some other countries. 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 UK switched from the long scale to the short scale.