tetrad

Noun

tetrad (plural tetrads)

1. A group of four things.
   • Religious movements and cults are often founded on a tetrad of elements: a prophet, a prophecy, a book, and a revelation.2010, Siddhartha Mukherjee, The Emperor of All Maladies, Fourth Estate (2011), page 177

   Synonyms:

   foursome, quartet, quadruplet, quadruplicity, quadrad, quaternion, quaternity, tetraplet

2. Two pairs of sister chromatids (a dyad pair) aligned in a certain way and often on the equatorial plane during the meiosis process.
3. A group of four haploid and immature pollen grains in tetrahedral fashion produced by meiotic microsporogenesis.
4. A unit of land area of two by two (that is, four) square kilometres.
   • 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.2010, Richard Mabey, Weeds: How Vagabond Plants Gatecrashed Civilisation and Changed the Way We Think about Nature, London: Profile Books, →ISBN
5. A tetravalent atom or radical.
6. A group of four basis vectors for a four-dimensional manifold in differential geometry.
7. A chord comprised of four notes; a tetrachord.