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.
The Century Dictionary and Cyclopedia
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;
the GNU version of the Collaborative International Dictionary of English
noun.
A word used to modify the sense of a verb, participle, adjective, or other adverb, and usually placed near it