tetrad

tetr- +‎ -ad

tetrad (plural tetrads)

1. A group of four things.
2. (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.

3. (chemistry) A tetravalent atom or radical.
4. (biology) Two pairs of sister chromatids (a dyad pair) aligned in a certain way and often on the equatorial plane during the meiosis process.
5. (biology) A group of four haploid and immature pollen grains in tetrahedral fashion produced by meiotic microsporogenesis.
6. (mathematics) A group of four basis vectors for a four-dimensional manifold in differential geometry.

• tetradic

• (a group of four things): quartet

• ratted, tarted