a group of four.
the number four.
Cell Biology. a group of four chromatids formed by synapsis at the beginning of meiosis.
Chemistry. a tetravalent or quadrivalent element, atom, or group.

1645–55; < Greek tetrad- (stem of tetrás) group of four

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World English Dictionary
tetrad (ˈtɛtræd)
1.  a group or series of four
2.  the number four
3.  botany a group of four cells formed by meiosis from one diploid cell
4.  genetics a four-stranded structure, formed during the pachytene stage of meiosis, consisting of paired homologous chromosomes that have each divided into two chromatids
5.  chem an element, atom, group, or ion with a valency of four
6.  ecology a square of 2 × 2 km used in distribution mapping
[C17: from Greek tetras, from tettares four]

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Medical Dictionary

tetrad tet·rad (tět'rād')

  1. A group or set of four.

  2. A tetravalent atom, radical, or element.

  3. A group of four chromatids formed from each of a pair of homologous chromosomes that split longitudinally during the prophase of meiosis.

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Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
tetrad   (tět'rād')  Pronunciation Key 
  1. A four-part structure that forms during prophase I of meiosis and consists of two homologous chromosomes, each composed of two identical chromatids. During prophase I of meiosis, one chromosome exchanges corresponding segments of genetic material with the other chromosome in the tetrad in the process called crossing over. See more at meiosis.

  2. A group of four cells, as of spores or pollen grains, formed from a parent cell by meiosis. As part of the process of spermatogenesis, a spermatocyte divides into a tetrad of four spermatids, cells which go on to develop into sperm. See more at spermatogenesis.

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