Intelligent memory polymer mask

An  was a kind of mask that Sarah Jane once found in the TARDIS in one of its many storage rooms. The Doctor claimed that Zoe had "picked it up from the Grand Festival of Zymymys Midamor" but that it was really "more the Master's kind of thing than [his]". Such masks were completely lifelike and even fleshy. The Doctor possessed at least two: one of James Stewart and another of Kim Novak. As the name implied, it was made of an intelligent memory polymer. (PROSE: Interference - Book Two)

Behind the scenes
The implication of the scene in Interference is that such masks were what enabled to realistically pass for various people — as was his frequent habit when played by Roger Delgado in such stories as The Mind of Evil. Paul Cornell would later suggest a different origin for the Master's masks in The Heralds of Destruction.