A pentagon is a five-sided polygon: it has one more side than a square.
This is a word you're most likely to learn in math class when studying geometry: a pentagon is a polygon that has five sides. There are many words for shapes that indicate their specific number of sides, such as octagon: a polygon with eight sides. The root penta means "five," which is why a pentagram has five points. You may have heard of the Pentagon, which is the center of the U.S. military in Washington D.C. It has this name because the building has five sides.