The DICT Development Group

  Tetrad \Tet"rad\, n. [L. tetras, -adis, Gr. ?, ?: cf. F.
     t['e]trade.]
     1. The number four; a collection of four things; a
        quaternion.
        [1913 Webster]
  
     2. (Chem.) A tetravalent or quadrivalent atom or radical; as,
        carbon is a tetrad.
        [1913 Webster] Tetradactyl

  tetrad
      n 1: the cardinal number that is the sum of three and one [syn:
           four, 4, IV, tetrad, quatern, quaternion,
           quaternary, quaternity, quartet, quadruplet,
           foursome, Little Joe]