An adverbial clause—also called a subordinate clause—is a dependent clause that functions as an adverb; that is, the entire clause modifies a verb, an adjective, or another adverb. As with all clauses, it contains a subject and predicate, although the subject as well as the (predicate) verb may sometimes be omitted and implied, see below.
An adverbial clause is usually, but not always, fronted by a subordinate conjunction—sometimes called a trigger word. (In the examples below the adverbial clause is italicized and the subordinate conjunction is bolded.)