adverb

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The part of speech that modifies a verb, an adjective, another adverb, or an entire clause or sentence.
• noun Any of the words belonging to this part of speech, such as so, very, and rapidly.

from The Century Dictionary.

• noun In grammar, one of the indeclinable parts of speech: so called from being ordinarily joined to verbs for the purpose of limiting or extending their signification, but used also to qualify adjectives and other adverbs: as, I readily admit; you speak wisely; very cold; naturally brave; very generally acknowledged;

from the GNU version of the Collaborative International Dictionary of English.

• noun (Gram.) A word used to modify the sense of a verb, participle, adjective, or other adverb, and usually placed near it

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun grammar A word that modifies a verb, adjective, other adverbs, or various other types of words, phrases, or clauses.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the word class that qualifies verbs or clauses
• noun a word that modifies something other than a noun