1. A group or set of four.
2. A tetravalent atom, radical, or element.
a. A four-part structure that forms during the prophase of meiosis and consists of two homologous chromosomes, each composed of two sister chromatids.
b. A group of four haploid cells, such as spores, formed by meiotic division of one mother cell.
[Greek tetras, tetrad-; see kwetwer- in the Appendix of Indo-European roots.]
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