Parts of speech serve our communication needs with hardly a conscious thought on our part, but they operate according to a complex, interdependent set of rules and procedures. Here are the basic principles of the noun.
A noun was traditionally described (at least, in the US public school system I oh so slowly passed through and briefly taught in) as a person, a place, or a thing, though some resources extend the definition to apply to intangible things — ideas such as peace and qualities such as fear — as separate categories.
Nouns are also subdivided into proper nouns and common nouns. Proper nouns refer to a specifically named entity such as, for example, a person named Mark, a place called California, a thing called a Mac, and an idea or quality personified as Perfection. Common nouns, by contrast, are generic: man, state, computer, perfection.
Three other classifications to distinguish nouns include countable and uncountable nouns, collective nouns, and concrete and abstract nouns. Countable nouns can be pluralized (word, words), can be accompanied by numbers or quantifiers (“Did he say, ‘Seven words’ or ‘several words’?), and can appear after an indefinite article (“In a word, yes”). Uncountable nouns (also called mass nouns and noncount nouns) share none of these qualities; examples include anger, geology, and weather. (Anger cannot be pluralized to angers, one would not say “seven geologies,” and weather would be preceded by a only if it is used as an adjective, as in “a weather system”).
Collective nouns are those denoting a single entity consisting of multiple components, such as team. Concrete nouns are those that name a physical entity or something experienced by operation of one or more senses (computer, article), while abstract ones refer to concepts or ideas (democracy, liberty).
Some nouns have both concrete and abstract senses; for example, dance refers to physical movement usually performed to music as well as a social event at which such activity occurs (and has a figurative meaning of “a careful interaction,” as in “The two superpowers exchanged messages in a delicate diplomatic dance”), but the word also denotes an art form involving often rhythmic and patterned physical movement.
A noun serves one of three syntactical functions in a sentence, all of which are demonstrated in the first clause of this sentence: the subject of a clause (“A noun”), the object of a verb (“serves . . . functions”), or the object of a preposition (“in a sentence”). Nouns, in English, are marked by their association with an article (a, an, the, and sometimes some), such as in the phrase “an article” here, or with one or more attributive adjectives. (In the preceding phrase — and a few words farther along in this sentence — attributive itself is an attributive adjective for adjective.)
A noun is also distinguished as the head, or key word, of a noun phrase; in this clause, clause is the head of the noun phrase “in this clause” and head is the head of the phrase “the head of the noun phrase”; note that that phrase has two nouns (head and phrase), but the latter is subordinate to the former.
Some grammar theories hold that a single word can constitute a noun phrase — and it doesn’t have to be a noun; consider the function of it in “It is true.”
A noun consisting of an adjective used in noun form is known as a nominalization; the last word in the phrase “a word to the wise” is an example.
Nouns are not essential to isolated sentences — “Yes,” for example, can constitute a sentence — but they are integral to communication.