Definition of mask:
(n) :
a
cover, or partial cover, for the
face, used for disguise or protection; that which
disguises; a pretext or subterfuge
"a dancer's mask; a fencer's mask; a ball player's mask"
(v) :
to
cover, as the face, by way of
concealment or defense against injury; to conceal with a mask or visor; to
disguise; to cover; to hide
"to mask a body of troops or a fortess by a superior force, while some hostile evolution is being carried out"