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  1. In geometry, a diagonal is a line that joins any two vertices of a polygon, if the vertices are not next to each other; or a line that joins two vertices of a polyhedron that are not on the same face.

       • GEOMETRY

  2. In a telescope, a diagonal is the small flat mirror used near the upper end of a Newtonian telescope to direct the converging beam of light over to the side of the tube where the eyepiece is located. See also star diagonal.