Mask

A mask was an object that completely covered someone's face, frequently to conceal identity.

Uses
The Units wore masks that connected them as a group mind. Akimus Makassar's mask worked as a processing centre, which meant that he could control the will of anyone in the group mind. (PROSE: The Masks of Makassar)

Magnus Greel and Elizabeth X wore masks to hide their appearances. (TV: The Talons of Weng-Chiang, The Beast Below)

Scaroth disguised his various fragments in human form behind masks. (TV: City of Death)

Roboforms wore masks as part of their Santa Claus guises. (TV: The Christmas Invasion, The Runaway Bride)

Some Silurian warriors wore masks. (TV: The Hungry Earth / Cold Blood, et al)

The Soulless wore masks as a representation of their belief that Miracle Day had robbed humanity of their souls. (TV: Dead of Night)

Voord masks altered the wearers genetic code and turned them into a Voord, provided they were willing to go through with the ritual. The mask became part of the wearer in the process and removing it could lead to disfigurement or more likely dead. (AUDIO: Domain of the Voord)

Other references
Joy believed that a Silent looked the way it did because it wore a Star Trek mask. (TV: The Impossible Astronaut) Similarly, a hospital porter in Cardiff believed a Weevil's face was a convincing mask. (TV: Everything Changes)

Behind the scenes
The BBC Doctor Who website featured several cut-out masks of aliens from the franchise that could be printed. Masks of varying materials were also available as free gifts in Doctor Who Adventures.