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. The term common name is not much used to contrast with proper name, but some linguists have used it for that purpose. While proper names are sometimes called simply names, this term is often used more broadly: "An earlier name for tungsten was wolfram." Words derived from proper names are occasionally called proper adjectives (or proper adverbs, and so on), but not in mainstream linguistic theory. Not every noun phrase that refers to a unique entity is a proper name. For example, chastity is a common noun even though chastity is considered a unique abstract entity (constrasted with the personal name Chastity, which is a proper name).
