One capable of appearing as someone else, especially many someones, without being recognized by people who know him.
This typically involves some change of appearance: A different hair style, fake but realistic mustache, makeup, different clothes, as well as props, such as glasses, canes, etc.
However, it always must involve a change of demeanor: Different posture, body language, speech patterns (including realistic accents), and things like that.
The personality change is more important than the appearance -- just think of any good actor not changing his external appearance, yet offering a completely different performance.
Fictional examples: Artemus Gordon, The Saint.