A phrase is a group of words that work together to express a unified meaning but that lack a subject, a predicate, or both. Each phrase contains one central element whose meaning may be built upon or modified by the other elements in the phrase. The central element determines the type of phrase: nouns are used to build noun phrases, verbs are used to build verb phrases, adjectives are used to build adjective phrases, and so on.
A noun phrase consists of the central noun or pronoun and all its modifiers, including determiners, adjectives, and adjective clauses.
Half of the world's population of nearly six billion people prepare their food and heat their homes with coal and the traditional biomass fuels of dung, crop residues, wood and charcoal.[The phrase in bold type is the noun phrase.]
--Daniel Kammen, "Cookstoves for the Developing World," Scientific American (modified)
A verb phrase consists of the central verb, any auxiliary verbs, any modifiers of the verb, and any direct objects or indirect objects.
The Large Hadron Collider at CERN, the European laboratory for particle physics near Geneva, will probably be built in the first few years of the 21st century.[The phrase in bold type is the verb phrase.]
--"Low-Energy Ways to Observe High-Energy Phenomena," Scientific American
An adjective phrase consists of the central adjective and any modifiers, including other adjectives or adverbs.
Beverage cans have emerged as the most important market for
--William Hostold and John Duncan, "The Aluminum Beverage Can," Scientific American (modified)
A prepositional phrase consists of the preposition and the noun phrase that follows.
Astrophysicists of the early 20th century, not knowing about nuclear
processes, computed that a sun powered by chemical burning or gravitational
shrinking could shine only for a few million years.
--"The Earth's Elements," Scientific American
Because phrases are used to express unified meanings, they should not be carelessly split or separated by other phrases or parts of other phrases.
See also Appositive Phrases and Participial Phrases.