Source : Webster's Revised Unabridged Dictionary (1913)

Octad \Oc"tad\, n. [Gr. ?, ?, the number eight.] (Chem.)
   An atom or radical which has a valence of eight, or is
   octavalent.

Source : WordNet®

octad
     n : the cardinal number that is the sum of seven and one [syn: {eight},
          {8}, {VIII}, {eighter}, {eighter from Decatur}, {ogdoad},
          {octonary}, {octet}]