tetrad

noun

1. 1.A group of four things.
2. 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. 3.A group of four haploid and immature pollen grains in tetrahedral fashion produced by meiotic microsporogenesis.
4. 4.A unit of land area of two by two (that is, four) square kilometres.
5. 5.A tetravalent atom or radical.