A hyperbolic version of the Euclidean Tetrahedron.
See also Hyperbolic Cube, Hyperbolic Dodecahedron, Hyperbolic Octahedron
References
Rivin, I. ``Hyperbolic Polyhedron Graphics.''
http://www.mathsource.com/cgi-bin/MathSource/Applications/Graphics/3D/0201-788.