tetrad

noun7

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
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 group of four basis vectors for a four-dimensional manifold in differential geometry.
5. •
   A chord comprised of four notes; a tetrachord.
6. •
   A tetravalent atom or radical.
7. •
   A unit of land area of two by two (that is, four) square kilometres.