A three-dimensional figure bounded by polygons. Each of its sides is called a face. Each of the straight lines which describe the meeting of faces is called an edge, and each point at the end of an edge is called a vertex. The plural form can be either polyhedrons or polyhedra.

For example, a tetrahedron is a polyhedron with four, two-dimensional faces, six edges, and six vertices. It is the simplest of all polyhedrons, and is a type of pyramid. The sides of a right tetrahedron are identical equilateral triangles. The regular tetrahedron is one of the five Platonic solids (along with the hexahedron (cube), octahedron, dodecahedron, and icosahedron). The plural form can be either tetrahedrons or tetrahedra. Here's a link to an animated image of a tetrahedron. (Artlex.com)

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