phrase

In everyday speech, a phrase may refer to any group of words. In linguistics, a phrase is a group of words which form a constituent and so function as a single unit in the syntax of a sentence. A phrase is lower on the grammatical hierarchy than a clause[1].

For example, the house at the end of the street is a phrase. It acts like a noun. It can further be broken down into two shorter phrases functioning as adjectives: at the end and of the street, a shorter prepositional phrase within the longer prepositional phrase. At the end of the street could be replaced by an adjective such as nearby: the nearby house or even the house nearby. The end of the street could also be replaced by another noun, such as the crossroads to produce the house at the crossroads.

Most phrases have an important word defining the type and linguistic features of the phrase. This word is the head of the phrase and gives its name to the phrase category[2]. For example the phrase the massive dinosaur is a noun phrase because its head word (dinosaur) is a noun. The head can be distinguished from its dependents (the rest of the phrase other than the head) because the head of the phrase determines many of the grammatical features of the phrase as a whole.
Categories of phrases

Phrases may be classified by the type of head taken by them:

Prepositional phrase (PP) with a preposition as head (e.g. in love, over the rainbow). Languages using postpositions instead have postpositional phrases. The two types are sometimes commonly referred to as appositional phrases.
Noun phrase (NP) with a noun as head (e.g. the black cat, a cat on the mat)
Verb phrase (VP) with a verb as head (e.g. eat cheese, jump up and down)
Appositive It renames noun as a pronoun and is always placed between commas (e.g. "Bob, my annoying neighbor, is short")
Absolute Modifies the entire sentence and is linked with commas. (e.g. "Mike threw the book, his eyes red")
Complexity

A complex phrase consists of several words, whereas a simple phrase consists of only one word. This terminology is especially often used with verb phrases:

simple past and present are simple phrases, which require just one verb
complex verbs have one or two aspects added and hence require additional two or three words

"Complex," which is phrase-level, is often confused with "compound", which is word-level. However, there are certain phenomena that formally seem to be phrases but semantically are more like compounds, such as "women's magazines," which has the form of a possessive noun phrase, but which refers (just like a compound) to one specific lexeme (i.e. a magazine for women and not a magazine owned by a woman).
Semiotic approaches to the concept of "phrase"

In more semiotic approaches to language, such as the more cognitivist versions of construction grammar, a phrasal structure is not only a certain formal combination of word types whose features are inherited from the head. Here each phrasal structure also expresses some type of conceptual content, be it specific or abstract.
References

^ Kroeger, Paul (2005). Analyzing Grammar: An Introduction. Cambridge: Cambridge University Press. pp. 35. ISBN 978-0-521-01653-7.
^ Kroeger, Paul (2005). Analyzing Grammar: An Introduction. Cambridge: Cambridge University Press. pp. 37. ISBN 978-0-521-01653-7.
adapted from: en.wikipedia.org/wiki/Phrase