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