tetrad

1. (biology) Two pairs of sister chromatids (a dyad pair) aligned in a certain way and often on the equatorial plane during the meiosis process.

2. (biology) A group of four haploid and immature pollen grains in tetrahedral fashion produced by meiotic microsporogenesis.

3. (cartography) A unit of land area of two by two (that is, four) square kilometres.Examples: "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."

4. (chemistry) A tetravalent atom or radical.

5. (mathematics) A group of four basis vectors for a four-dimensional manifold in differential geometry.

6. (music) A chord comprised of four notes; a tetrachord.