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").

Entries linking to tetrad

*kwetwer- 
Proto-Indo-European root meaning "four."

It forms all or part of: cadre; cahier; carillon; carrefour; catty-cornered; diatessaron; escadrille; farthing; firkin; fortnight; forty; four; fourteen; fourth; quadrant; quadraphonic; quadratic; quadri-; quadrilateral; quadriliteral; quadrille; quadriplegia; quadrivium; quadroon; quadru-; quadruped; quadruple; quadruplicate; quarantine; quarrel (n.2) "square-headed bolt for a crossbow;" quarry (n.2) "open place where rocks are excavated;" quart; quarter; quarterback; quartermaster; quarters; quartet; quarto; quaternary; quatrain; quattrocento; quire (n.1) "set of four folded pages for a book;" squad; square; tessellated; tetra-; tetracycline; tetrad; tetragrammaton; tetrameter; tetrarch; trapezium.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit catvarah, Avestan čathwaro, Persian čatvar, Greek tessares, Latin quattuor, Oscan petora, Old Church Slavonic četyre, Lithuanian keturi, Old Irish cethir, Welsh pedwar.
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Definitions of tetrad

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

Dictionary entries near tetrad

tetherball

Tethys

Teton

tetra-

tetracycline

tetrad

tetragrammaton

tetrahedron

tetralogy

tetrameter

tetrapod

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