Simplex

In geometry, a simplex (plural: "simplices" or "simplexes"), sometimes called "n-simplex" is the "n"-dimensional analogue figure of the triangle. A triangle is a 2-simplex. The 0-simplex is the point, the 1-simplex is the line segment (line connecting two points). The 3-simplex is the tetrahedron, and the 4-simplex is the pentachoron.
A regular simplex is a simplex that is also a regular polytope. A regular "n"-simplex may be made from a regular ("n" - 1)-simplex by connecting a new vertex to all the other vertices. All the vertices have the same length.