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1. an angle of a polygon contained between one side extended and the adjacent side
2. any of the four angles made by a transversal that are outside the region between the two intersected lines
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
exterior angle[ek′stir·ē·ər ′aŋ·gəl]
An angle between one side of a polygon and the prolongation of an adjacent side.
An angle made by a line (the transversal) that intersects two other lines, and either of the latter on the outside.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.