The Collaborative International Dictionary of English v.0.48:

Heptad \Hep"tad\, n. [L. heptas the number seven. Gr. ?, ?, fr.
   "epta` seven.] (Chem.)
   An atom which has a valence of seven, and which can be
   theoretically combined with, substituted for, or replaced by,
   seven monad atoms or radicals; as, iodine is a heptad in
   iodic acid. Also used as an adjective.
   [1913 Webster]

WordNet (r) 3.0 (2006):

heptad
    n 1: the cardinal number that is the sum of six and one [syn:
         seven, 7, VII, sevener, heptad, septet,
         septenary]