| Definition -
Any group of words that function together as a single part of speech.
1. As opposed to a clause, which is a group of words that has both a predicate and a (sometimes implied) subject.
In the sentence I didn't know that the dog ran through the yard
(a) that the dog ran through the yard is a clause, as is the sentence as a whole, and
(b) the yard, through the yard, ran through the yard, and the dog are all phrases
(c) the store across the street is a noun phrase because it starts with a noun,
(d) across the street is a prepositional phrase because it begins with the preposition across,
(e) run fast is a verb phrase because it starts with a verb,
(f) green grass is an adjectival phrase because it starts with an adjective; and
(g) very carefully is an adverbial phrase because it starts with the adverb very.
Etymology - The word derives via Late Latin from the Greek phrasis, speech or way of speaking.
Oxford English Dictionary - Its first citation in this sense is from 1852:
"The predicate may be extended in various ways:—1. By an adverb, or an adverbial phrase."
(Morell Anal. Sent. §17)