The word is silentium.
To mime is to act out silently.
The word silently *is* an adverb. The adjective form is "silent."
silently
more silently
Silently is an adverb. Many adverbs end in the prefix -LY.
cheeky
Noisily
creeps
It is neither. The word silently is an adverb, the adverb form of the adjective silent.
Yes, it is the adverb form of the adjective silent.
The word silently, like most (but not all) words that end in -ly, is an adverb.
si-lent-ly