Source : Webster's Revised Unabridged Dictionary (1913)

Hexad \Hex"ad\, n. [L. hexas, hexadis, the number six, Gr. ?, ?,
   fr. "e`x six.] (chem.)
   An atom whose valence is six, and which can be theoretically
   combined with, substituted for, or replaced by, six monad
   atoms or radicals; as, sulphur is a hexad in sulphuric acid.
   Also used as an adjective.

Source : WordNet®

hexad
     n : the cardinal number that is the sum of five and one [syn: {six},
          {6}, {VI}, {sixer}, {sise}, {Captain Hicks}, {half a
         dozen}, {sextet}, {sestet}, {sextuplet}]