Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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