Source : Webster's Revised Unabridged Dictionary (1913)

Tetrad \Tet"rad\, n. [L. tetras, -adis, Gr. ?, ?: cf. F.
   t['e]trade.]
   1. The number four; a collection of four things; a
      quaternion.

   2. (Chem.) A tetravalent or quadrivalent atom or radical; as,
      carbon is a tetrad.

Source : WordNet®

tetrad
     n : the cardinal number that is the sum of three and one [syn: {four},
          {4}, {IV}, {quatern}, {quaternion}, {quaternary}, {quaternity},
          {quartet}, {quadruplet}, {foursome}, {Little Joe}]