1 : a covering for your face or for part of your face: such as
a : a covering used to hide or disguise your face
a Halloween mask
Everyone wore costumes and masks to the dance.
The robbers wore masks.
b : a covering used to protect your face or cover your mouth
Doctors wear surgical masks in the operating room.
a (baseball) catcher's mask
2 : a way of appearing or behaving that is not true or real — usually singular
Her anger and resentment were hidden behind a mask of friendship. [=she pretended to be friendly but was really angry and resentful]