“adverb: A word of a class traditionally defined as a modifying a verb, e.g. badly in He wrote it badly, seen as a modifier of wrote.
One of the parts of speech established in antiquity. In the grammar of English, the words called adverbs are in practice those whose primary roles is a s modifier of something other than a noun. Thus an adverb such as utterly modifies a verb or verb phrase in They destroyed it utterly, and an adjective in This is utterly crazy. Very modifies an adjective, as in a very big house, or an adverb, as in very badly. Then has its primary role in e.g. I did it then, though it can also modify a noun, e.g. in her then husband. The case for lumping such words together is that many have been formed with the suffix -ly, and their roles often overlap.”
Excerpted from: Matthews, P.H., ed. The Oxford Concise Dictionary of Linguistics. New York: Oxford University Press, 2014.