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.

Not all nouns that refer to a unique entity are proper nouns. Blackness and chastity are common nouns, even if blackness and chastity are considered unique abstract entities.

Proper names are distinguished from proper nouns by some linguists. When that distinction is made, proper nouns are limited to single words only (possibly with "the"), while proper names include all proper nouns (in their primary applications) as well as noun phrases such as United Kingdom, North Carolina, Royal Air Force, and the White House.United Kingdom, for example, is a proper name with the common noun kingdom as its head, and North Carolina is headed by the proper noun Carolina. Especially as titles of works, but also as nicknames and the like, some proper names contain no noun and are not formed as noun phrases (the film Being There; Hi De Ho as a nickname for Cab Calloway and as the title of a film about him).