[1] : 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

[2] : The Collaborative International Dictionary of English v.0.48

pentad \pen"tad\, a. chem.
   having the valence of a pentad.
   1913 webster pentadactyl

[3] : WordNet (r) 2.0

pentad
     n : the cardinal number that is the sum of four and one syn: five,
          5, v, cinque, quint, quintet, fivesome, quintuplet,
          fin, phoebe, little phoebe
see also:
five 5 v cinque quint quintet 
fivesome quintuplet fin phoebe little phoebe 

Pentad >> P Words
Pentad, definition of term: Pentad
pentad_pag1.html