• A pyramid is a solid whose base is a polygon (a triangle, a square, a pentagon, and so on), and whose lateral faces are triangles that all meet at one point called the apex.
• The perpendicular line from the apex to the plane of the base is the height of the pyramid, written h.
• If the foot of this perpendicular is the centre of the base, the pyramid is called a right pyramid.

The regular pyramid

• A regular pyramid is a right pyramid whose base is a regular polygon, that is, all its sides are equal and all its angles are equal.
• In a regular pyramid all the lateral faces are congruent isosceles triangles.
• The height of a lateral face, measured from the apex to the midpoint of a side of the base, is called the slant height of the pyramid, written l.