tetrahedron

A polyhedron with four faces.

A solid comprehended under four plane faces; especially, the regular tetrahedron, or triangular pyramid having its base and sides equilateral triangles.

A solid figure inclosed or bounded by four triangles.

A solid bounded by four equal equilateral triangles; one of the five regular solids.

A polyhedron with four faces; the regular tetrahedron, the faces of which are equal equilateral triangles, is one of the Platonic solids.