Mirror

Mirrors were polished surfaces (usually glass coated with silver) which cast reflections. Some mirrors exhibited unusual properties.

Overview
In 1866, Edward Waterfield and Theodore Maxtible experimented with static electricity and mirrors in an attempt to develop time travel. (DW: The Evil of the Daleks) Another time machine that used mirrors was developed by General Mariah Learman in the early 21st century. (BFA: The Time of the Daleks)

In an alternate timeline, UNIT used mirrors and the Doctor's TARDIS to create a Circle of Mirrors and send Donna Noble back in time. (DW: Turn Left)

The Tharils could use mirrors to travel to different dimensions (DW: Warriors' Gate), as could the Mirrorlings. (DWAM: Mirror Image)

Mirrors were harmful to some life forms, and were used to defeat the Gorgon on Zeno (TVC: The Gaze of the Gorgon), another Gorgon on Earth (SJA: Eye of the Gorgon), Medusa in the Land of Fiction (DW: The Mind Robber), and the Mara on Deva Loka. (DW: Kinda) The Borad banned mirrors during his reign over Karfel. (DW: Timelash)

The Tenth Doctor punished Daughter of Mine by trapping her within every mirror. She was said to be the strange movement one sees out of the corner of one's eye when looking in a reflective surface. (DW: The Family of Blood)

The legendary Mortal Mirror of Castle Extremis was a portal to a parallel universe. Those who passed through it and were observed by someone on their side became trapped in the other realm, only able to return to their own reality as fragile beings made entirely out of glass. (NSA: Martha in the Mirror)

The Saturnynians' use of perception filters meant that mirrors would not show their reflections (DW: The Vampires of Venice), whereas the Krafayis could only be seen in mirrors. (DW: Vincent and the Doctor)

Susan discovered a full-length mirror in the TARDIS wardrobe room which reflected a pale-skinned young man with fangs. (ST: The Exiles) Later, Ace discovered this same mirror and the same reflection of the young man. (ST: Echo)