The Collaborative International Dictionary of English v.0.48:

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

WordNet (r) 3.0 (2006):

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]