Source : Webster's Revised Unabridged Dictionary (1913)

Pentagonal \Pen*tag"o*nal\, a. [Cf. F. pentagonal, pentagone, L.
   pentagonus, pentagonius, Gr. ?.]
   Having five corners or angles.

   {Pentagonal dodecahedron}. See {Dodecahedron}, and
      {Pyritohedron}.