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 10<sup>9</sup> years. This initialism is often used in the sciences of astronomy, geology, and paleontology.
The "billion" in bya is the 10<sup>9</sup> "billion" of the short scale of the US, not the long-scale 10<sup>12</sup> "billion" of some European usage. Billion by this convention (10<sup>9</sup>) 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.