Definitions of nouns:
Source: Wikipedia
Nouns have sometimes been defined in terms of the grammatical categories to which they are subject (classed by gender, inflected for case and number). Such definitions tend to be language-specific, since nouns do not have the same categories in all languages.
Nouns are frequently defined, particularly in informal contexts, in terms of their semantic properties (their meanings). Nouns are described as words that refer to a person, place, thing, event, substance, quality, quantity, etc. However this type of definition has been criticized by contemporary linguists as being uninformative.[6]
Linguists often prefer to define nouns (and other lexical categories) in terms of their formal properties. These include morphological information, such as what prefixes or suffixes they take, and also their syntax – how they combine with other words and expressions of particular types. Such definitions may nonetheless still be language-specific, since syntax as well as morphology varies between languages. For example, in English it might be noted that nouns are words that can co-occur with definite articles (as stated at the start of this article), but this would not apply in Russian, which has no definite articles.
There have been several attempts, sometimes controversial, to produce a stricter definition of nouns on a semantic basis. Some of these are referenced in the Further reading section below.
Forms of nouns:

A noun in its basic form will often consist of a single stem, as in the case of the English nouns cat, man, table and so on. In many languages nouns can also be formed from other nouns and from words of other types through morphological processes, often involving the addition of prefixes and suffixes. Examples in English are the verbal nouns formed from verbs by the addition of -ing, nouns formed from verbs using other suffixes such as organization and discovery, agent nouns formed from verbs usually with the suffix -er or -or, as in actor and worker, feminine forms of nouns such as actress, lioness, nouns formed from adjectives such as happiness, and many other types.
Nouns may be identical in form to words that belong to other parts of speech, often as a result of conversion (or just through coincidence). For example the English word hit can be both a noun and a verb, and the German Arm/arm can be a noun or an adjective. In such cases the word is said to represent two or more lexemes.
In many languages nouns inflect (change their form) for number, and sometimes for case. Inflection for number usually involves forming plural forms, such as cats and children (see English plural), and sometimes other forms such as duals, which are used in some languages to refer to exactly two of something. Inflection for case involves changing the form of a noun depending on its syntactic function – languages such as Latin, Russian and Finnish have extensive case systems, with different forms for nominatives (used principally for verb subjects), accusatives (used especially for direct objects), genitives (used to express possession and similar relationships) and so on. The only real vestige of the case system on nouns in Modern English is the “Saxon genitive”, where ‘s is added to a noun to form a possessive.

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