What does "mask" mean?

Additional senses

1. 2.A festive entertainment of dancing or other diversions, where all wear masks; a masquerade; hence, a revel; a frolic; a delusive show. Bacon. This thought might lead me through the world's vain mask. Milton.
2. 3.A dramatic performance, formerly in vogue, in which the actors wore masks and represented mythical or allegorical characters.
3. 4.(Arch.) A grotesque head or face, used to adorn keystones and other prominent parts, to spout water in fountains, and the like; -- called also mascaron.
4. 5.(Fort.) (a) In a permanent fortification, a redoubt which protects the caponiere. (b) A screen for a battery.
5. 6.(Zoöl.) The lower lip of the larva of a dragon fly, modified so as to form a prehensile organ. Mask house, a house for masquerades. [Obs.]
6. 7.To cover, as the face, by way of concealment or defense against injury; to conceal with a mask or visor. They must all be masked and vizarded. Shak.
7. 8.To disguise; to cover; to hide. Masking the business from the common eye. Shak.
8. 9.(Mil.) (a) To conceal; also, to intervene in the line of. (b) To cover or keep in check; as, to mask a body of troops or a fortess by a superior force, while some hostile evolution is being carried out.
9. 10.To take part as a masker in a masquerade. Cavendish.
10. 11.To wear a mask; to be disguised in any way. Shak.