Etymology
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tetrad (n.)

"the number four, collection of four things," 1650s, from Greek tetras (combining form tetrad-) "group of four, number four" (from PIE root *kwetwer- "four").

updated on October 09, 2017

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Definitions of tetrad from WordNet

tetrad (n.)
the cardinal number that is the sum of three and one;
Synonyms: four / " / iv / quatern / quaternion / quaternary / quaternity / quartet / quadruplet / foursome / Little Joe
From wordnet.princeton.edu, not affiliated with etymonline.