What part of speech is stillness?

Still can be used as a noun, verb, adjective, or adverb.

The still of the night somehow seemed ominous. (noun)

The police seized the illegal still. (slang noun: alcohol distillation equipment)

His assurances could not still the protests from the miners. (verb)

The still body of a fallen rebel lay over the cannon. (adjective)

The scientists took a video and several still photographs. (adjective)

The rain was still falling the next morning. (adverb)

The igloo was drafty, but it was still better than no shelter at all. (adverb)