A pyramid is a polyhedron built by connecting a polygonal base with a point termed the apex in geometry. A lateral face is formed by the base edge and apex of each triangle. Pyramids are all self-dual. The peak of a right pyramid is immediately above the centroid of its base. The apex of a left pyramid is below its base, on the opposite side. The word "apex" comes from a Greek word meaning "point of departure". In mathematics, an apex is a point at which a plane can be considered to make a definite departure from a surface; for example, the apex of a mountain is where it becomes invisible beneath its own shadow. In geometry, the apex of a pyramid is the point where another plane intersects the base of the pyramid. Thus, the apex of a pyramid is the point at which two planes of different nature (the base and the apical) meet. In mathematics and physics, the centroid of a convex polygon is the center of mass of the polygon. In mathematics, the centroid of a pyramid is the intersection of its base with any perpendicular drawn from its apex to the ground. The term "centroid" comes from a Greek word meaning "center". In English, the word "centre" has been used for some time now to describe the point around which a circle revolves. But in mathematics, a centre always denotes a point.