From 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]
From 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