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.Also see mathematics, polygon, and vertex.