Hi Richard,

The word vertex is defined in out online glossary as

vertex (plural: vertices)

* the point of intersection of two rays that form an angle, two sides of a polygon or two edges of a solid.

This definition refers to the term vertex when applied to a polygon or a polyhedron. The term is used more widely in particular with reference to a cone where the vertex is the point at the apex of the cone.

A hemisphere has no vertices.

I hope this helps,
Harley