From Teflpedia

A phrase (/freɪz/) is [better definition needed]. It acts as a single grammatical unit in a sentence.

A single-word phrase consists of a single word, whereas a multi-word phrase consists of multiple words.

A phrase has a head which determines its syntactic properties. Hence noun phrase, adjective phrase, adverb phrase, verb phrase, prepositional phrase, wh- phrase. And finally, adverbial.