The name comes from the Egyptian square-based pyramids.
But in general a pyramid is a polyhedron formed by joining
the edges of a polygon to a point to form sloping triangular
faces.

The volume of any pyramid is equal to 1/3 Ah, where
A is the area of its base polygon and h is its height. Pyramids
are generally classified by their bases.

For example, the
Egyptian pyramids have square bases, and are therefore called
square pyramids. A triangular pyramid is also known as a tetrahedron
(four sides).

Example of a Pyramid: