1. A polygon having five sides and five interior angles.
2. Pentagon A five-sided building near Washington, D.C., containing the U.S. Department of Defense and the offices of the U.S. Armed Forces. Used with the.

[Late Latin pentagōnum, from Greek pentagōnon : penta-, penta- + -gōnon, -gon.]

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.