In astronomy, geology, and paleontology, Gya or Ga (for giga years ago and gigaannum, respectively) is a year multiplied by one of the SI prefix multipliers. It is often used as a unit of time to denote length of time before the present. Specifically, one Gya is equal to 109 (1,000,000,000) years ago.
A unit identical to Gya is bya (for "billion years ago"), which may be in more widespread use. However, some scientists prefer Gya because it uses the SI prefix giga- and thus avoids confusion over whether the short-scale (American) billion (109) (1,000,000,000) or long-scale (former British) billion (1012) (1,000,000,000,000) is meant, as well as avoiding preference for one of the two definitions of billion over the other.
In some scientific literature, the term Gya has been replaced by Ga (Giga-annum).
Gya can also refer to a new unit of radiation exposure called the gray. A 100 röntgen dose of radiation is equivalent to 1 Gya, the gray dose in air.