definition

Geometry, a kind of cone, consists of four triangles, also called tetrahedron.

The base is a regular triangle, and the projection of the vertex on the base is a triangular pyramid at the center of the base triangle.

Called regular triangular pyramid; A tetrahedron composed of four congruent regular triangles is called a regular tetrahedron.

(A regular triangular pyramid is different from a regular tetrahedron. Every face of a regular tetrahedron must be a regular triangle. )