From The Collaborative International Dictionary of English v.0.48:
Mask \Mask\ (m[.a]sk), n. [F. masque, LL. masca, mascha, mascus; cf. Sp. & Pg. m['a]scara, It. maschera; all fr. Ar. maskharat buffoon, fool, pleasantry, anything ridiculous or mirthful, fr. sakhira to ridicule, to laugh at. Cf. Masque, Masquerade.] 1. A cover, or partial cover, for the face, used for disguise or protection; as, a dancer's mask; a fencer's mask; a ball player's mask. [1913 Webster]
2. That which disguises; a pretext or subterfuge. [1913 Webster]
3. A festive entertainment of dancing or other diversions, where all wear masks; a masquerade; hence, a revel; a frolic; a delusive show. --Bacon. [1913 Webster]
This thought might lead me through the world's vain mask. --Milton. [1913 Webster]
4. A dramatic performance, formerly in vogue, in which the actors wore masks and represented mythical or allegorical characters. [1913 Webster]
5. (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. [1913 Webster]
6. (Fort.) (a) In a permanent fortification, a redoubt which protects the caponiere. (b) A screen for a battery. [1913 Webster]
7. (Zool.) The lower lip of the larva of a dragon fly, modified so as to form a prehensile organ. [1913 Webster]
8. A person wearing a mask; a masker.
The mask that has the arm of the Indian queen. --G. W. Cable. [Webster 1913 Suppl.]
9. (Sporting) The head or face of a fox.
Mask house, a house for masquerades. [Obs.]
Death mask, a cast of the face of a dead person. [1913 Webster]
From The Collaborative International Dictionary of English v.0.48:
Mask \Mask\, v. t. [imp. & p. p. Masked; p. pr. & vb. n. Masking.] 1. To cover, as the face, by way of concealment or defense against injury; to conceal with a mask or visor. [1913 Webster]
They must all be masked and vizarded. --Shak. [1913 Webster]
2. To disguise; to cover; to hide. [1913 Webster]
Masking the business from the common eye. --Shak. [1913 Webster]
3. (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 fortress by a superior force, while some hostile evolution is being carried out. [1913 Webster]
From The Collaborative International Dictionary of English v.0.48:
Mask \Mask\, v. i. 1. To take part as a masker in a masquerade. --Cavendish. [1913 Webster]
2. To wear a mask; to be disguised in any way. --Shak. [1913 Webster]