Adverb

Adverb refers a word which modifies a verb, an adjective or another adverb.

Comments

In the syntax, adverbs such as quickly, or probably, can also modify VPs, or sentences. It has been proposed that adverbs form a distinct category, ADV, because - unlike the lexical categories - they do not project a phrase in the sense of X-bar-theory. Since adverbs never occur as a complement, they are treated as adjuncts.

Link

Utrecht Lexicon of Linguistics