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Etymology edit

From Ancient Greek τετράς (tetrás), analysable as tetra- +‎ -ad.

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tetrad (plural tetrads)

  1. A group of four things.
    Synonyms: foursome, quartet; see also Thesaurus:quartet
  2. (biology) Two pairs of sister chromatids (a dyad pair) aligned in a certain way and often on the equatorial plane during the meiosis process.
  3. (biology) A group of four haploid and immature pollen grains in tetrahedral fashion produced by meiotic microsporogenesis.
  4. (cartography) A unit of land area of two by two (that is, four) square kilometres.
    • 2010, Richard Mabey, Weeds: How Vagabond Plants Gatecrashed Civilisation and Changed the Way We Think about Nature, London: Profile Books, →ISBN:
      They took figures for the abundance of invasive weeds mapped according to the normal grid unit of the 'hectad', or 10 × 10 km square, and then looked at how abundant these species were mapped at a much finer scale – in 'tetrads', or 2 × 2 km squares, inside these hectads.
  5. (chemistry) A tetravalent atom or radical.
  6. (mathematics) A group of four basis vectors for a four-dimensional manifold in differential geometry.

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