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In astronomy, geology, and paleontology, "bya" or "b.y.a." is an initialism for " billion years ago". This abbreviation is commonly used as a unit of time to denote length of time before the present in billions of years.

The "billion" in bya is, by convention, the 10 "billion" of the short scale of the U.S., not the long-scale 10 "billion" of some European usage. 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.