A polyhedron (one polyhedron, many polyhedra, or polyhedrons) is a geometrical shape. It is a 3D shape with flat faces, and straight edges. Each face is a polygon surrounded by edges. Usually it is defined by the number of faces, or edges.
Two types of polyhedron are convex and concave. The line connecting any two points of a convex polyhedron is inside the polyhedron. The line connecting two points of a concave polyhedron may go outside the polyhedron.
Mathematicians do not agree on what makes a polyhedron. They do agree that there are five Platonic solids, which are regular polyhedra. On a regular polyhedron, all faces are the same and all vertices are the same.