The second mirror in a reflecting telescope (after the primary mirror), usually either convex, to reflect the image out of a hole in the bottom of the telescope to the Cassegrain focus or along the telescope mount axis to the coudé focus; or flat, to reflect the image out of the side of the telescope to the Newtonian focus.
A small mirror user to direct the incoming image from the primary system to the eyepiece in an optical system.
In a two-mirror reflecting telescope, the secondary mirror sits in front of the larger primary mirror and reflects light to the point at which it will be detected and recorded by an instrument. In simple telescopes, the secondary mirror is flat and bounces the light out the side of the tube to an eyepiece. In more complex and larger telescopes, it is convex and reflects light through a hole in the primary mirror.