It is actually somewhat inaccurate to label a noun as definite or indefinite in its own right, for nouns do not exhibit these qualities in themselves, but only in the context in which they are used when they are combined with an article or a demonstrative adjective.

We indicate that a singular noun has a definite meaning by adding the definite article “the” or the demonstrative “this” or “that” to it. When we do so, our audience knows that we are talking about a particular individual when we use the noun. For instance, when we say “the cat,” we are talking about a specific cat. When we say “that cat” or “this cat,” we are also being definite in our expression. If we are talking about more than one cat and want to express ourselves in a definite way, we would say “those cats” or “these cats,” for we have some specific cats in mind.

If we are not thinking in terms of a particular individual example of a given noun, however, we are expressing ourselves in an indefinite manner. A singular noun is marked as indefinite by adding the indefinite article “a” or “an.” If we want to talk about the idea of “cat” in general, we would say something like, “A cat is able to sleep anywhere.” We are not thinking about a particular cat but rather making a general statement. The plural indicative expression lacks an article. We say, for instance, “Cats always land on their feet,” when we want to talk about more than one cat in an indefinite way.

The supporting words 'the' and 'a' point to the difference between a definite and indefinite singular noun. A noun that is definite is associated with a particular object or person. 'The"'vase means, for instance, that the speaker is referring to one specific vase out of all the vases in the universe. 'A' vase means any vase out of a number of vases and can be used to refer to the general concept of "vaseness": you could, for example, say that "a vase is usually made of porcelain," denoting the category of vases rather than one particular vase.

We can expand our definition outward to encompass more definite articles, such as this, that, and those. These words concretize and specify a particular thing: 'that' woman, like 'the' woman, is not any woman, but a specific person. 'Those' teenagers are particular group of teenagers. In contrast, 'a' woman is much more hazy and indefinite. Saying I saw 'a' woman could mean any woman; 'the' woman, in contrast, always has some sort of known referent behind it, such as 'the' woman who said she would call, who stole the vase, or who saved my cat. There is no indefinite article in the plural, so the plural noun alone denotes the quality of being indefinite: for example, "teenagers milled around" is indefinite, while "those teenagers milled around" is definite.

This concept can be confusing and seem superfluous to non-native speakers whose language of origin does not include articles, but articles are integral to spoken and written English.

Generally, the concepts of definite and indefinite in the English language functions to remove ambiguity or confusion, but it is imperfect in its ability to clarify.

A definite noun in English is usually preceded by the definite article "the," removing any ambiguity in terms of who or what is being referred to. For example, the definite noun phrase "the waiter" refers to a specific person, as opposed to the indefinite noun phrase containing the indefinite article "a" which introduces ambiguity. For example, "a waiter" could refer to any waiter and therefore is nonspecific. However, if a diner has not yet met anyone on the waitstaff and says "the waiter will take care of our drinks," the waiter is being referred to in the abstract—not a specific person—even with the definite article "the."

What delineates indefinite from definite nouns or articles is something that philosopher Bertrand Russell calls "uniqueness" in an essay that includes an argument about the role of definite determiners. Russell's argument includes ideas that go beyond the pragmatics of grammatical elements that prevent confusion about case.

Definite nouns refer to a specific number of things. For example, a definite noun is "bird." When readers come across this noun, they are sure of how many birds the sentence, or phrase, is referring to. Given that the word "bird" is singular, a reader knows that it refers to one bird. On the other hand, when one comes across the word "birds," he or she is not certain of the number; therefore, the noun is indefinite. The reader is only aware that more than one bird is referring to, given the word "birds" is plural. One must also be sure to examine the article ("a" and "the"). "A" is an indefinite article (it refers to a general, not specific, noun), while "the" is a definite article (given it refers to a specific noun).