Tetrahedron
or TETRAE- DRON, in geometry, a polyhedron bounded by four triangles. If the middle points of the faces be properly joined, two and two, the lines joining them are TETRAHEDRON the edges of a second tetrahedron. A regular tetrahedron is one in which the faces are equal and equilateral triangles.
If the middle points of the faces be joined two and two, the lines joining them form the edges of a regular tetrahedron. All regular tetrahedrons are similar solids.