Byr

The abbreviation byr means "billion years" (109 or 1,000,000,000 years). A closely related abbreviation is bya "billion years ago".

Byr was formerly used in English-language geology and astronomy as a unit of one billion years. Today, the term gigaannum (Ga) is also used, but Gy or Gyr are still sometimes used in English-language works (at the risk of confusion with the gray). Astronomers use Gyr as an abbreviation for Gigayear. Specifically, one Gyr is equal to 109 years.

Because a billion means 1000 million in some countries but can mean a million million in others its use is deprecated in favour of giga- which unambiguously means 109 and annum which for precision may be qualified by (sidereal), (Julian), etc.