pentad

from 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}]

from 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]

from The Collaborative International Dictionary of English v.0.48

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

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