Source : Webster's Revised Unabridged Dictionary (1913)

Pentavalent \Pen*tav"a*lent\, a. [Penta- + L. valens, p. pr. See
   {Valence}.] (Chem.)
   Having a valence of five; -- said of certain atoms and
   radicals.

Source : WordNet®

pentavalent
     adj : having a valence of five