Mask refers to a covering for all or part of the face, worn as a disguise, or to protect or conceal the face.
In medical context, mask specifically means a protective covering for the nose and mouth to prevent the spread of germs.
The term "mask" also refers to:
1. A facial pack or beauty treatment applied to the skin
2. In computing, a bit pattern used to control the modification of another bit pattern
3. To conceal or disguise something