Bya

{{Short description|Abbreviation for "billion years ago"}}

{{Other uses|BYA (disambiguation)}}

{{lowercase}}

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.,Gale, Joseph (2009). Astrobiology of Earth: The Emergence. Oxford: Oxford University Press, p. [https://books.google.com/books?id=64zn0nxDVUYC&pg=PA21 21.] 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.Chartrand, Mark (2004). Satellite Communications for the Nonspecialist. New York: SPIE Press, p. [https://books.google.com/books?id=MM0d2cMUWbEC&pg=PA73 73.] 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.

Related units are mya ("million years ago"), and byr ("billion years"). These are traditionally written in lowercase. Ga or Gya has a capitalized first letter instead.