Definition of Proper noun

A proper noun is a noun that in its primary application refers to a unique entity, such as London, Jupiter, Sarah, or Toyota, as distinguished from a common noun, which usually refers to a class of entities (cities, planets, persons, corporations), or non-unique instances of a certain class (a city, another planet, these persons, our corporation). Some proper nouns occur in plural form, and then they refer to groups of entities considered as unique (the Hendersons, The Azores, the Pleiades). Proper nouns can also occur in secondary applications, for example modifying nouns (the Mozart experience; his Azores adventure), or as de facto common nouns (he's no Pavarotti; they drive Toyotas). The detailed definition of the term is problematic and to an extent governed by convention.