Source : Webster's Revised Unabridged Dictionary (1913)

Adverb \Ad"verb\, n. [L. adverbium; ad + verbum word, verb: cf.
   F. adverbe.] (Gram.)
   A word used to modify the sense of a verb, participle,
   adjective, or other adverb, and usually placed near it; as,
   he writes well; paper extremely white.

Source : WordNet®

adverb
     n 1: the word class that qualifies verbs or clauses
     2: a word that modifies something other than a noun