Real Dictionary

adverb
countable noun: An adverb is a word such as 'slowly', ' now', 'very', 'politically', or ' fortunately' which adds information about the action, event, or situation mentioned in a clause.

adverb in American English
any of a class of words used generally to modify a verb, an adjective, another adverb, a phrase, or a clause, by expressing time, place, manner, degree, cause, etc.: English adverbs often end in -ly (Ex.: fast, carefully, then)
noun: any member of a class of words that in many languages are distinguished in form, as partly in English by the ending -ly, or by functioning as modifiers of verbs or clauses, and in some languages, as Latin and English, also as modifiers of adjectives or ot

adverb in British English
noun: a word or group of words that serves to modify a whole sentence, a verb, another adverb, or an adjective; for example, probably, easily, very, and happily respectively in the sentence They could probably easily envy the very happily married couple