GENERAL: A word which modifies a verb, an adjective or another adverb.
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.