Defining a Noun Can Be Difficult

In The Teacher's Grammar Book (2005), James Williams admits that "defining the term noun is such a problem that many grammar books do not even try to do it." Interestingly, however, one of the founders of cognitive linguistics has settled on a familiar definition:

In elementary school, I was taught that a noun is the name of a person, place, or thing. In college, I was taught the basic linguistic doctrine that a noun can only be defined in terms of grammatical behavior, conceptual definitions of grammatical classes being impossible. Here, several decades later, I demonstrate the inexorable progress of grammatical theory by claiming that a noun is the name of a thing.
(Ronald W. Langacker, Cognitive Grammar: A Basic Introduction. Oxford University Press, 2008)

Professor Langacker notes that his definition of thing "subsumes people and places as special cases and is not limited to physical entities."

It's probably impossible to come up with a universally accepted definition of noun. Like many other terms in linguistics, its meaning depends on context and use as well as the theoretical biases of the person doing the defining. So rather than wrestle with competing definitions, let's just briefly consider some of the conventional categories of nouns — or more precisely, some of the different ways of grouping nouns in terms of their (often overlapping) forms, functions, and meanings.

For additional examples and more detailed explanations of these slippery categories, follow the links to our Glossary of Grammatical and Rhetorical Terms.

Abstract Nouns and Concrete Nouns
An abstract noun is a noun that names an idea, quality, or concept (courage and freedom, for example).
A concrete noun is a noun that names a material or tangible object — something recognizable through the senses (such as chicken and egg).
But this apparently simple distinction can get tricky. Lobeck and Denham point out that "the classification of a noun can change depending on how that noun is used and what it's referring to in the real world. When homework refers to the idea of schoolwork that will be completed over time, it seems more abstract, but when it refers to an actual document that you submit for a class, it seems concrete" (Navigating English Grammar, 2014).
Attributive Nouns
An attributive noun is a noun that serves as an adjective in front of another noun--such as "nursery school" and "birthday party."
Because so many nouns can serve as adjective equivalents, it's more accurate to regard attributive as a function than as a type. The clustering of nouns in front of another noun is sometimes called stacking.

Collective Nouns
A collective noun is a noun that refers to a group of individuals — such as team, committee, and family.
Either a singular or a plural pronoun can stand in for a collective noun, depending on whether the group is regarded as a single unit or as a collection of individuals. (See Pronoun Agreement.)
Common Nouns and Proper Nouns
A common noun is a noun that's not the name of any particular person, place, or thing (for instance, singer, river, and tablet).
A proper noun is a noun that refers to a specific person, place, or thing (Lady Gaga, Monongahela River, and iPad).
Most proper nouns are singular, and — with a few exceptions (iPad) — they're usually written with initial capital letters. When proper nouns are used generically (as in "keeping up with the Joneses" or "a xerox of my term paper"), they become, in a sense, common — and in some cases subject to lawsuits. (See Generification.)
Count Nouns and Mass Nouns
A count noun is a noun that has both singular and plural forms — like dog(s) and dollar(s).
A mass noun (also called a noncount noun) is a noun that's generally used only in the singular and can't be counted — music and knowledge, for instance.
Some nouns have both countable and non-countable uses, such as the countable "dozen eggs" and the non-countable "egg on his face."

Denominal Nouns
A denominal noun is a noun that's formed from another noun, usually by adding a suffix--such as guitarist and spoonful.
But don't count on consistency. While a librarian usually works in a library and a seminarian usually studies in a seminary, a vegetarian can show up anywhere. (See Common Suffixes in English.)
Verbal Nouns
A verbal noun (sometimes called a gerund) is a noun that's derived from a verb (usually by adding the suffix -ing) and that exhibits the ordinary properties of a noun — for example, "My mother didn't like the idea of my writing a book about her."
Most contemporary linguists distinguish verbals from deverbals, but not always in precisely the same way.