The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

hexad
    n 1: 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, hexad]