A proper noun is a noun that identifies a single entity and is used to refer to that entity (Africa; Jupiter; Sarah; Walmart) 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.
