A mirror is a piece of highly reflective material.
History and specifics
In 2266, on the planet Fayo, Fayons illuminated the jungle without the use of electricity. Ten flaming torches produced light, which was then reflected by an enormous mirror to generate a spotlight. Camelephants pulled the large contraption. (TOS comic: "Prince Traitor")
In 2270, Doctor Leonard McCoy used an antique mirror, which was used by doctors in the past to determine if a patient was alive, to show Shaun Christopher that he was in James T. Kirk's body. (TOS novel: The Rings of Time)
In 2273, Anton Zauber used a forming pod to alter his physical appearance into that of Doctor Leonard McCoy. When the pod was opened by a medical droid, his wife Joann Zauber held up a mirror, so that he could see how well the procedure worked. (TOS comic: "The Real McCoy")