The Collaborative International Dictionary of English v.0.48:

Octad \Oc"tad\, n. [Gr. ?, ?, the number eight.]
   1. (Chem.) An atom or radical which has a valence of eight,
      or is octavalent.
      [1913 Webster]

   2. A group of eight things.
      [PJC]

WordNet (r) 3.0 (2006):

octad
    n 1: the cardinal number that is the sum of seven and one [syn:
         eight, 8, VIII, eighter, eighter from Decatur,
         octad, ogdoad, octonary, octet]