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.
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.