A pyramid is a polyhedron in which one face (the base) can be any polygon, and the other faces (the lateral faces) are triangles that meet at a common vertex called the vertex of the pyramid. Similar to the altitude of a triangle, the altitude of a pyramid is the segment from the vertex that is perpendicular to the plane of the base.

The length of the altitude is called the height of the pyramid. A pyramid is regular if its base is a regular polygon and its lateral faces are congruent, isosceles triangles. In a regular pyramid, the length of the altitude of each lateral face is sometimes referred to as the slant height of a pyramid .

If a pyramid is not regular, it is called irregular or oblique.