Byr was formerly used in English-language geology and astronomy as a unit of one billion years. The "B" is an abbreviation for "billion" (meaning thousand million), with "yr" simply an abbreviation for "year". 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). Some scientists 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.

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