A square based pyramid is a pentahedron, meaning it has five faces. It consists of four equilateral triangles, which have equal sides and equal angles. This shape is a regular polygon, which by definition has a regular polygon as its base and equally sized triangles as its faces. An irregular polygon has an irregular polygon as its base and faces of different sizes.
Other types of pyramids include a triangular pyramid, pentagonal pyramid and hexagonal pyramid, all of which are named for the shape of their bases. A right pyramid has sides that are isosceles triangles, which have two equal sides and angles. An oblique pyramid has at least one side that is not an isosceles triangle.Learn more about Shapes