Tetrad

noun

1. A group of four things.
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.
5. A tetravalent atom or radical.