'Non' is a prefix or adverb meaning 'not' or 'the opposite of' (as in 'non-existent,' 'non-stop'). When used as a standalone word, it functions as an adverb indicating negation. Adverbs modify verbs, adjectives, or other adverbs. 'Non' is not a verb, noun, pronoun, or preposition - it's an adverbial particle indicating negation.