A proper noun is a noun that identifies a single entity and is used to refer to that entity (Africa, Jupiter, Sarah, Microsoft) as distinguished from a common noun, which is a noun that refers to a class of entities (continent, planet, person, corporation) and may be used when referring to instances of a specific class (a continent, another planet, these persons, our corporation). Some proper nouns occur in plural form (optionally or exclusively), and then they refer to groups of entities considered as unique (the Hendersons, the Everglades, the Azores, the Pleiades). Proper nouns can also occur in secondary applications, for example modifying nouns (the Mozart experience; his Azores adventure), or in the role of common nouns (he's no Pavarotti; a few would-be Napoleons). The detailed definition of the term is problematic and, to an extent, governed by convention.
A distinction is normally made in current linguistics between proper nouns and proper names. By this strict distinction, because the term noun is used for a class of single words (tree, beauty), only single-word proper names are proper nouns: Peter and Africa are both proper names and proper nouns; but Peter the Great and South Africa, while they are proper names, are not proper nouns (though they could be said to function as proper noun phrases). The term common name is not much used to contrast with proper name, but some linguists have used the term for that purpose. Sometimes proper names are called simply names, but that term is often used more broadly. Words derived from proper names are sometimes called proper adjectives (or proper adverbs, and so on), but not in mainstream linguistic theory. Not every noun or a noun phrase that refers to a unique entity is a proper name. Chastity, for instance, is a common noun, even if chastity is considered a unique abstract entity.
Few proper names have only one possible referent: there are many places named New Haven; Jupiter may refer to a planet, a god, a ship, a city in Florida, or a symphony; at least one person has been named Mata Hari, but so have a horse, a song, and three films; there are towns and people named Toyota, as well as the company. In English, proper names in their primary application cannot normally be modified by articles or another determiner, although some may be taken to include the article the, as in the Netherlands, the Roaring Forties, or the Rolling Stones. A proper name may appear to have a descriptive meaning, even though it does not (the Rolling Stones are not stones and do not roll; a woman named Rose is not a flower). If it had once been, it may no longer be so, for example, a location previously referred to as "the new town" may now have the proper name Newtown, though it is no longer new and is now a city rather than a town.
In English and many other languages, proper names and words derived from them are associated with capitalization; but the details are complex, and vary from language to language (French lundi, Canada, un homme canadien, un Canadien; English Monday, Canada, a Canadian man, a Canadian; Italian lunedì, Canada, un uomo canadese, un canadese). The study of proper names is sometimes called onomastics or onomatology, while a rigorous analysis of the semantics of proper names is a matter for philosophy of language.
Proper nouns are normally invariant for number: most are singular, but a few, referring for instance to mountain ranges or groups of islands, are plural (e.g. Hebrides). Typically, English proper nouns are not preceded by an article (such as the or a) or other determiner (such as that or those).
Occasionally, what would otherwise be regarded as a proper noun is used as a common noun, in which case a plural form and a determiner are possible. Examples are in cases of ellipsis (for instance, the three Kennedys = the three members of the Kennedy family) and metaphor (for instance, the new Gandhi, likening a person to Mahatma Gandhi).