dihedral angle: it is each of the two parts of space bounded by two half-planes that start from a common edge. It is an ideal geometrical concept, and may represent only part, as two rectangles with a common side, which symbolize two half-planes.
Face: is each of the planes forming a dihedral angle or polyhedron, or each of the polygons forming a polyhedron or limit.
Concave: When at least one of the interior angles of a polygon is greater than 180 °, it is a concave polygon. The inner surface is concave bowl when viewed from above.
Cube or hexahedron regular: a polyhedron of six congruent square faces, one of the so-called Platonic solids.
Dodecahedron: a twelve-sided polyhedron, convex or concave. Their faces are to be polygon of eleven sides or less. If the twelve faces of the dodecahedron are regular pentagons necessarily equal to each other, the dodecahedron is convex and is called regular.
Icosahedron: a twenty-sided polyhedron, convex or concave. Their faces are to be nineteen-sided polygons or less. If the twenty faces are triangles of the icosahedron equilateral necessarily equal to each other, the icosahedron is convex and is called regular, then being a so-called Platonic solids. The conjugate polyhedron of the icosahedron is the dodecahedron.
Octahedron: a polyhedron with eight faces. With this number of faces can be a convex polyhedron or a concave polyhedron. Their faces are to be seven-sided polygons or less.
Tetrahedron: a polyhedron with four faces. With this number of heads is bound to be a convex polyhedron, and triangular faces, three of them were found at each vertex. If the four faces of the tetrahedron are equilateral triangles, necessarily equal to each other, the tetrahedron is called regular.
Polyhedron: is, in the sense given by classical geometry at the end, a solid figure whose faces are flat and contain a finite volume. The word polyhedron comes from the Greek word classic πολύεδρον, poly-many-sided edron.
convex polyhedron: the one in which the segment joining any two points is contained in the polyhedron.
Prism: it is a solid finish by two parallel and equal polygons which are called bases and many sides are parallelograms as the foundation, called faces.
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