The Collaborative International Dictionary of English v.0.48:

Pentad \Pen"tad\, n. [Gr. penta`s penta`dos, a body of five, fr.
   pe`nte five.] (Chem.)
   Any element, atom, or radical, having a valence of five, or
   which can be combined with, substituted for, or compared
   with, five atoms of hydrogen or other monad; as, nitrogen is
   a pentad in the ammonium compounds.
   [1913 Webster]

   2. Any grouping of five things.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

Pentad \Pen"tad\, a. (Chem.)
   Having the valence of a pentad.
   [1913 Webster] Pentadactyl

WordNet (r) 3.0 (2006):

pentad
    n 1: the cardinal number that is the sum of four and one [syn:
         five, 5, V, cinque, quint, quintet, fivesome,
         quintuplet, pentad, fin, Phoebe, Little Phoebe]