tetrahedron - any polyhedron having four plane faces pyramid - a polyhedron having a polygonal base and triangular sides with a common vertex OR pyramid Pronunciation (pr-md) n. 1. a. A solid figure with a polygonal base and triangular faces that meet at a common point. b. Something shaped like this polyhedron. 2. a. A massive monument of ancient Egypt having a rectangular base and four triangular faces culminating in a single apex, built over or around a crypt or tomb. b. Any of various similar constructions, especially a four-sided Mesoamerican temple having stepped sides and a flat top surmounted by chambers. 3. The transactions involved in pyramiding stock. 4. Anatomy A structure or part suggestive of a pyramid in shape. tetrahedron Pronunciation (ttr-hdrn) n. pl. tetrahedrons or tetrahedra (-dr) A polyhedron with four faces.