In syntax and grammar, a phrase is a group of words which act together as a grammatical unit. For instance, the English language expression "the very happy squirrel" is a noun phrase which contains the adjective phrase "very happy". Phrases can consist of a single word or a complete sentence. In theoretical linguistics, phrases are often analyzed as units of syntactic structure such as a constituent.
In theories of syntax, a phrase is any group of words, or sometimes a single word, which plays a particular role within the syntax structure of a sentence. It does not have to have any special meaning or significance, or even exist anywhere outside of the sentence being analyzed, but it must function there as a complete grammatical unit. For example, in the sentence Yesterday I saw an orange bird with a white neck, the words an orange bird with a white neck form a noun phrase, or a determiner phrase in some theories, which functions as the object of the sentence.
There are two competing principles for constructing trees; they produce 'constituency' and 'dependency' trees and both are illustrated here using an example sentence. The constituency-based tree is on the left and the dependency-based tree is on the right:
The tree on the left is of the constituency-based, phrase structure grammar, and the tree on the right is of the dependency grammar. The node labels in the two trees mark the syntactic category of the different constituents, or word elements, of the sentence.
In the constituency tree each phrase is marked by a phrasal node (NP, PP, VP); and there are eight phrases identified by phrase structure analysis in the example sentence. On the other hand, the dependency tree identifies a phrase by any node that exerts dependency upon, or dominates, another node. And, using dependency analysis, there are six phrases in the sentence.
The trees and phrase-counts demonstrate that different theories of syntax differ in the word combinations they qualify as a phrase. Here the constituency tree identifies three phrases that the dependency tree does not, namely: house at the end of the street, end of the street, and the end. More analysis, including about the plausibilities of both grammars, can be made empirically by applying constituency tests.
In the following phrases the head-word, or head, is bolded:
The above five examples are the most common of phrase types; but, by the logic of heads and dependents, others can be routinely produced. For instance, the subordinator phrase:
By linguistic analysis this is a group of words that qualifies as a phrase, and the head-word gives its syntactic name, "subordinator", to the grammatical category of the entire phrase. But this phrase, " before that happened", is more commonly classified in other grammars, including traditional English grammars, as a subordinate clause (or dependent clause); and it is then labelled not as a phrase, but as a clause.
Another type is the inflectional phrase, where (for example) a finite verb phrase is taken to be the complement of a functional, possibly covert head (denoted INFL) which is supposed to encode the requirements for the verb to inflection – for agreement with its subject (which is the specifier of INFL), for tense and aspect, etc. If these factors are treated separately, then more specific categories may be considered: tense phrase (TP), where the verb phrase is the complement of an abstract "tense" element; aspect phrase; agreement phrase and so on.
Further examples of such proposed categories include topic phrase and focus phrase, which are argued to be headed by elements that encode the need for a constituent of the sentence to be marked as the topic or focus.
The distinction is illustrated with the following examples:
The syntax trees of this sentence are next:
The constituency tree on the left shows the finite verb string may nominate Newt as a constituent; it corresponds to VP1. In contrast, this same string is not shown as a phrase in the dependency tree on the right. However, both trees, take the non-finite VP string nominate Newt to be a constituent.