A Tetrad

1. A group of four things. Synonyms: foursome, quartet
2. (biology) Two pairs of sister chromatids (a dyad pair) aligned in a certain way and often on the equatorial plane during the meiosis process.
3. (biology) A group of four haploid and immature pollen grains in tetrahedral fashion produced by meiotic microsporogenesis.
4. (cartography) A unit of land area of two by two (that is, four) square kilometres.
5. (chemistry) A tetravalent atom or radical.
6. (mathematics) A group of four basis vectors for a four-dimensional manifold in differential geometry.