A
proper noun is a
noun that in its primary application refers to a unique entity, such as
London,
Jupiter,
Sarah, or
Microsoft, as distinguished from a
common noun, which usually refers to a
class of entities (
city, planet, person, corporation), or non-unique instances of a specific class (a
city, 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.