vertical angle definition by Webster's New World
Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
either of a pair of angles of equal degree, lying opposite to each other at the intersection of two straight lines
vertical angle definition by American Heritage Dictionary
Either of two angles formed by two intersecting lines and lying on opposite sides of the point of intersection.