An adverbial is a syntagma which has the same distribution as an adverb. Just like other syntactic constructions, adverbials are classified by their internal structure and by their function.

By its internal structure, an adverbial may or may not have an adverb as its head. If it is thus headed, it may reduce to its head or may be expanded. If it reduces to its head, it may be a pro-adverb or a lexical adverb. If it is not a headed phrase, it may be a recategorized syntagma of a different category. Worthy of especial note are noun phrases and subordinate clauses functioning as adverbials.

By its syntactic function, an adverbial is, by default (and true to its name) an adjunct (a modifier of a verb). Otherwise, it may modify an element of a different category, e.g. a noun (as an adverbial attribute).