a government building with five sides that serves as the headquarters of the United States Department of Defense
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the United States military establishment
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a five-sided polygon
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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.