We call it *polyhedron* the solid bounded by four or more flat polygons, belonging to different planes and having two by two only one edge in common. See some examples:

Polygons are the faces of polyhedron; the sides and vertices of polygons are the edges and vertices of the polyhedron.

## Convex and concave polyhedra

Looking at the polyhedra above, we can see that considering any of its faces, the polyhedra are entirely in the same semi-space as this face determines. So these polyhedra are called *convex*.

This is not the case in the last polyhedron, because in relation to two of its faces it is not contained only in a semi-space. Therefore, it is termed concave.

## Classification

Convex polyhedra have special names according to the number of faces, such as:

tetrahedron: four faces

pentahedron: five faces

hexahedron: six faces

heptahedron: seven faces

octahedron: eight faces

icosahedron: twenty faces