From Longman Dictionary of Contemporary Englishpentagonpentagon noun [countable] CFa flat shape with five sides and five angles —pentagonal /penˈtæɡənəl/ adjective
Examples from the Corpuspentagon• It is easy to see that there are 12 pentagons on the ball.• They are completely regular solids: each has all of its faces repeating a regular polygon, triangle, square or pentagon.• Two methods for construction of the regular pentagon.• The regular pentagon, in association with the hexagon, has been credited with magical powers throughout recorded history.• A group of six pentagons folds naturally to form a serrated hemisphere.• Folding of square, pentagon, ellipse, spider and tetrahedron.• All the magic of the pentagon is transferred to this solid.• For example, the square has four-fold symmetry, the pentagon has five-fold, the hexagon has six-fold symmetry.Pentagon, thethe PentagonPentagon, the the building in Washington DC from which the army, navy etc of the US are controlled, or the military officers who work in this building. It is called the Pentagon because it has a five-sided shape. On 11 September 2001 it was attacked by terrorists who flew a plane filled with passengers into it. 189 people were killed, including the 64 people who were on the plane.