The name is derived from Greek roots, and so the name, not surprisingly, is "tetrahedron".
Tetrahedron. http://en.wikipedia.org/wiki/Tetrahedron
"Tetra" is the Greek word for "four," and the word "hedron" translates to "face." So a tetrahedron has four faces, all of which are triangles.
A tetrahedron.
tetrahedron
A tetrahedron.
A triangular pyramid.
tetrahedron
Tetrahedron
A tetrahedron.
A tetrahedron.
a tetrahedron
A triangle-based pyramid.