NOUN (3)

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;

The Collaborative International Dictionary of English v.0.48:

Pentagon \Pen"ta*gon\, n. [Gr. penta`gwnon; penta- (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.
   [1913 Webster]

   Regular pentagon, a pentagon in which the angles are all
      equal, and the sides all equal.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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