Adjunct adverbials are words, phrases, and clauses that modify an entire clause by providing additional information about time, place, manner, condition, purpose, reason, result, and concession.
Adjunct adverbial is a grammatical function.
The grammatical forms that can function as the adjunct adverbial in English grammar are the adverb phrase, prepositional phrase, adverb clause, noun phrase, and verb phrase.
Adjunct adverbials are not constituents of either the subject or the predicate.
Brinton, Laurel J. & Donna M. Brinton. 2010. The linguistic structure of Modern English, 2nd edn. Amsterdam: John Benjamins Publishing Company.