A noun phrase is either a single noun or pronoun or a group of words containing a noun or a pronoun that function together as a noun or pronoun, as the subject or object of a verb.
Examples of noun phrases:
EG: John was late.
('John' is the noun phrase functioning as the subject of the verb.)
EG: The people that I saw coming in the building at nine o'clock have just left.
('The people ... nine o'clock' is a lengthy noun phrase, but it functions as the subject of the main verb 'have just left'.)
Noun phrases normally consist of a head noun, which is optionally modified ("pre modified" if the modifier is placed before the noun; "post modified" if the modifier is placed after the noun). Possible modifiers include:
Determiners: articles (the, a), demonstratives (this, that), numerals (two, five, etc.), possessives (my, their, etc.), and quantifiers (some, many, etc.). In English, determiners are usually placed before the noun;
Adjectives (the red ball); or
Complements, in the form of a prepositional phrase (such as: the student of physics), or a That-clause (the claim that the earth is round);
Modifiers; pre-modifiers if placed before the noun and usually either as nouns (the university student) or adjectives (the beautiful lady), or post-modifiers if placed after the noun. A post modifier may be either a prepositional phrase (the man with long hair) or a relative clause (the house where I live). The difference between modifiers and complements is that complements complete the meaning of the noun; complements are necessary, whereas modifiers are optional because they just give additional information about the noun.
Noun phrases can make use of an opposition structure. This means that the elements in the noun phrase are not in a head-modifier relationship, but in a relation of equality. An example of this is I, Caesar, declare ..., where "Caesar" and "I" do not modify each other.
The head of a noun phrase can be implied, as in "The Bold and the Beautiful" or Robin Hood's "rob from the rich and give to the poor"; an implied noun phrase is most commonly used as a generic plural referring to human beings. Another example of noun phrase with implied head is I choose the cheaper of the two.
That noun phrases can be headed by elements other than nouns—for instance, pronouns (They came) or determiners (I'll take these)—has given rise to the postulation of a determiner phrase instead of a noun phrase. The English language is not as permissive as some other languages, with regard to possible heads of noun phrases. German, for instance, allows adjectives as heads of noun phrases , as in Gib mir die Alten for Give me the olds (i.e. old ones).