Source : Webster's Revised Unabridged Dictionary (1913)

Pentad \Pen"tad\, n. [Gr. ?, ?, a body of five, fr. ? 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.

Pentad \Pen"tad\, a. (Chem.)
   Having the valence of a pentad.

Source : WordNet®

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}]