In geometry, a shape with five sides is called a pentagon. A regular pentagon has five sides of equal length, as well as five equal angles. A standard "STOP" sign in the US is a regular pentagon, as is the Pentagon building in the District of Columbia.
An irregular pentagon is any five-sided shape that does not have equal sides and angles. A convex pentagon has no angles greater than 180 degrees, which means all angles point outward. The "STOP" sign is an example of a convex pentagon. A concave pentagon has at least one angle over 180 degrees, or pointing inward.