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.

 

Star diagonal

A star diagonal is a gadget resembling a plumbing elbow fitting, containing a mirror or prism, attached to a downward-pointing focuser to redirect the beam of light upward for more convenient viewing through a telescope. That is, a star diagonal fits into a focuser, has an eyepiece fitted into it, and bends the light path through a right angle.