The number four; a collection of four things; a quaternion.
- A group or set of four.
- A tetravalent atom, radical, or element.
- Biology A four-part structure that forms during the prophase of meiosis and consists of two homologous chromosomes, each composed of two sister chromatids.
- Botany Biology A group of four cells, as of spores or pollen grains, formed by division of one mother cell.