Vertical Angles Definition
Vertical angles are angles that are opposite one another at the intersection of two lines. In other terms, given two intersecting lines, the two nonadjacent angles with the same vertex are said to be vertical angles. It is easy to demonstrate or prove that vertical angles are congruent. Some sources claim that the term vertical is used to denote that the angles share the same vertex while others use vertical to mean vertically opposite. As such, vertical angles are sometimes called opposite angles. In the diagram below, angles A and D are vertical.
“Vertical Angles.” From Wolfram MathWorld, mathworld.wolfram.com/VerticalAngles.html.