Question

vertex
a point at either end of a line segment, or a point at one end of a ray.
angles on the outside of parallel lines cut by a transversal
nonadjacent interior angles found on opposite sides of the transversal.
the common endpoint of an angle

Explanation:

The term "vertex" in geometry refers to the common endpoint of an angle. The first description is about endpoints of line - segments or rays, the second is about exterior angles related to parallel lines and transversals, and the third is about non - adjacent interior angles. But the key definition for vertex is the common endpoint of an angle.

Answer:

The common endpoint of an angle