A Metamorphmagus is a witch or wizard with the ability to change his or her physical appearance at will, rather than requiring Polyjuice Potion or a spell like the rest of the wizarding population. They are extremely rare.  Metamorphmagi can take a wide variety of forms, changing sex and age, potentially looking like anyone at all, or even just changing a part of their appearances, such as the shape of their noses.  They can remain in their altered state for several hours, if they wish to.