An adverbial phrase is a linguistic

Linguistics

Linguistics is the scientific study of human language. Linguistics can be broadly broken into three categories or subfields of study: language form, language meaning, and language in context....

 term for a group of two or more words operating adverbial

Adverbial

In grammar an adverbial is a word or a group of words that modifies or tells us something about the sentence or the verb. The word adverbial is also used as an adjective, meaning 'having the same function as an adverb'...

ly, when viewed in terms of their syntactic function

Syntax

In linguistics, syntax is the study of the principles and rules for constructing phrases and sentences in natural languages....

.

Compare the following sentences:

• I'll go to bed soon.
• I'll go to bed in an hour.
• I'll go to bed when I've finished my book.

In the first, soon is an adverb

Adverb

An adverb is a part of speech that modifies verbs or any part of speech other than a noun . Adverbs can modify verbs, adjectives , clauses, sentences, and other adverbs....

 (as distinct from a noun

Noun

In linguistics, a noun is a member of a large, open lexical category whose members can occur as the main word in the subject of a clause, the object of a verb, or the object of a preposition .Lexical categories are defined in terms of how their members combine with other kinds of...

 or verb

Verb

A verb, from the Latin verbum meaning word, is a word that in syntax conveys an action , or a state of being . In the usual description of English, the basic form, with or without the particle to, is the infinitive...

), and it is an adverbial

Adverbial

In grammar an adverbial is a word or a group of words that modifies or tells us something about the sentence or the verb. The word adverbial is also used as an adjective, meaning 'having the same function as an adverb'...

 (as distinct from a subject

Subject (grammar)

The subject is one of the two main constituents of a clause, according to a tradition that can be tracked back to Aristotle and that is associated with phrase structure grammars; the other constituent is the predicate. According to another tradition, i.e...

 or object

Object (grammar)

An object in grammar is part of a sentence, and often part of the predicate. It denotes somebody or something involved in the subject's "performance" of the verb. Basically, it is what or whom the verb is acting upon...

). Clearly, in the second sentence

Sentence (linguistics)

In the field of linguistics, a sentence is an expression in natural language, and often defined to indicate a grammatical unit consisting of one or more words that generally bear minimal syntactic relation to the words that precede or follow it...

, in an hour has the same syntactic function, though it does not contain an adverb; therefore, a prepositional phrase consisting of a preposition and a noun (preceded by its article

Article (grammar)

An article is a word that combines with a noun to indicate the type of reference being made by the noun. Articles specify the grammatical definiteness of the noun, in some languages extending to volume or numerical scope. The articles in the English language are the and a/an, and some...

) can function as an adverbial and is called an adverbial phrase. In the third sentence, we see a whole clause

Clause

In grammar, a clause is the smallest grammatical unit that can express a complete proposition. In some languages it may be a pair or group of words that consists of a subject and a predicate, although in other languages in certain clauses the subject may not appear explicitly as a noun phrase,...

 functioning as an adverbial; it is termed an adverbial clause

Adverbial clause

An adverbial clause is a dependent clause that functions as an adverb. In other words, it contains a subject and a predicate, and it modifies a verb.*I saw Joe when I went to the store....

.