Source : Webster's Revised Unabridged Dictionary (1913)

Pentagon \Pen"ta*gon\, n. [Gr. ?; ? (see {Penta-}) + gwni`a
   angle: cf. L. pentagonium, F. pentagone.] (Geom.)
   A plane figure having five angles, and, consequently, five
   sides; any figure having five angles.

   {Regular pentagon}, a pentagon in which the angles are all
      equal, and the sides all equal.

Source : WordNet®

Pentagon
     n 1: a government building with five sides that serves as the
          headquarters of the United States Department of Defense
     2: the United States military establishment
     3: a five-sided polygon