The definition of adverbial is to have the same function as an adverb (a word that is a verb, adjective or other adverb).(adjective)
An example of an adverbial phrase is a clause which describes a verb (i.e. to say “Before Aunt Mabel came over...” instead of “Yesterday”).
See adverbial in Webster's New World College Dictionary
See adverbial in American Heritage Dictionary 4
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