tetrad

noun

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.”

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.”

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.