<!-- Note Gigayear redirects here -->Byr was formerly used in English-languagegeology and astronomy as a unit of one billionyears. The "B" is an abbreviation for "billion" (meaning 1,000,000,000), 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). Astronomers use Gyr as an abbreviation for Gigayear. 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 10<sup>9</sup> and annum which for precision may be qualified by (sidereal), (Julian), etc.