A proper noun is a noun directly associated with an entity and primarily used to refer to that entity, such as London, Jupiter, Sharon, or Microsoft, as distinguished from a common noun, which is a noun directly associated with a class of entities (city, planet, person, corporation) and primarily used to refer to 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.
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 the term for that purpose. Sometimes proper names are called simply names; but that term is often used more broadly. Words derived from proper names are sometimes called proper adjectives (or proper adverbs, and so on), but not in mainstream linguistic theory. Not every noun or noun phrase that refers to a unique entity is a proper name. Blackness and chastity are common nouns, even if blackness and chastity are considered unique abstract entities.
Few proper names have only one possible referent: there are many places named New Haven; Jupiter may refer to a planet, a god, a ship, or a symphony; at least one person has been named Mata Hari, but so have a horse, a song, and three films; there are towns and people named Toyota, as well as the company.
In English, proper names in their primary application cannot normally be modified by an article or other determiner (such as any or another), although some may be taken to include the article the, as in the Netherlands, the Roaring Forties, or the Rolling Stones. A proper name may appear to refer by having a descriptive meaning, even though it does not (the Rolling Stones are not stones and do not roll; a woman named Rose is not a flower). Or if it had once been descriptive (and then perhaps not even a proper name at all), it may no longer be so (a location previously referred to as "the new town" may now have the proper name Newtown, though it is no longer new, and is now a city rather than a town).
In English and many other languages, proper names and words derived from them are associated with capitalization; but the details are complex, and vary from language to language (French lundi, Canada, canadien; English Monday, Canada, Canadian).
In linguistics, common nouns and proper nouns are distinct subclasses of nouns. A common noun refers to a class of entity (e.g. dog), whereas a proper noun refers to an individual entity (e.g. John or Kennedy) or a collection of entities (e.g. Hebrides). Proper nouns are normally invariant for number: most are singular, but a few, referring for instance to mountain ranges or groups of islands, are plural (e.g. Hebrides). Typically, English proper nouns are not preceded by an article (the or a) or other determiners (not, for instance, a John, the Kennedy, or many Hebrides).
Occasionally, what would otherwise be regarded as a proper noun is used as a common noun, in which case a plural form and a determiner are possible (for instance the three Kennedys, the new Gandhi).
Current linguistics makes a distinction between proper nouns and proper names,[a] but this distinction is not universally observed, and sometimes it is observed but not rigorously.[b] When the 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 the United Kingdom, North Carolina, Royal Air Force, and the White House.[c] Proper names can have a common noun or a proper noun as their head; the 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).
Proper names are also referred to (by linguists) as naming expressions. Sometimes they are called simply names;  but that term is also used more broadly (as in "chair is the name for something we sit on"); the latter type of name is called a common name to distinguish it from a proper name.
Common nouns are frequently used as components of proper names. Some examples are agency, boulevard, city, day, and edition. In such cases the common noun may determine the kind of entity, and a modifier determines the unique entity itself. For example:
Proper nouns, and all proper names, differ from common nouns grammatically. They may take titles, such as Mr Harris or Senator Harris. Otherwise, they normally only take modifiers that add emotive coloring, such as old Mrs Fletcher, poor Charles, or historic York; in a formal style, this may include the, as in the inimitable Henry Higgins. They may also take the in the manner of common nouns in order to establish the context in which they are unique: the young Mr Hamilton (not the old one), the Dr Brown I know; or as proper nouns to define an aspect of the referent: the young Einstein (Einstein when he was young). The indefinite article a may similarly be used to establish a new referent: the column was written by a [or one] Mary Price. Proper names based on noun phrases differ grammatically from common noun phrases. They are fixed expressions, and cannot be modified internally: beautiful King's College is acceptable, but not King's famous College.
As with proper nouns, so with proper names more generally: they may only be unique within the appropriate context. For instance, India has a ministry of home affairs (a common-noun phrase) called the Ministry of Home Affairs (its proper name). Within the context of India, this identifies a unique organization. However, other countries may also have ministries of home affairs called "the Ministry of Home Affairs", but each refers to a unique object, so each is a proper name. Similarly, "Beach Road" is a unique road, though other towns may have their own roads named "Beach Road" as well. This is simply a matter of the pragmatics of naming, and of whether a naming convention provides identifiers that are unique; and this depends on the scope given by context.
Because they are used to refer to an individual entity, proper names are, by their nature, definite; so a definite article would be redundant, and personal names (like John) are used without an article or other determiner. However, some proper names (especially certain geographical names) are usually used with the definite article. These have been termed weak proper names, in contrast with the more typical strong proper names, which are normally used without an article. Entities with weak proper names include geographical features (e.g. the Mediterranean, the Thames), buildings (e.g. the Parthenon), institutions (e.g. the House of Commons), cities and districts (e.g. The Hague, the Bronx), works of literature (e.g. the Bible), and newspapers and magazines (e.g. The Times, The Economist, the New Statesman). Plural proper names are weak. Such plural proper names include mountain ranges (e.g. the Himalayas), and collections of islands (e.g. the Hebrides).
The definite article is omitted when a weak proper noun is used attributively (e.g. "Hague residents are concerned ...", "... eight pints of Thames water ...").
Proper names often have a number of variants, for instance a formal variant (David, the United States of America) and an informal variant (Dave, the United States).
In languages that use alphabetic scripts and that distinguish lower and upper case, there is usually an association between proper names and capitalization. In German, all nouns are capitalized, but other words are also capitalized in proper names (not including composition titles), for instance: der Große Bär (the Great Bear, Ursa Major). For proper names, as for several other kinds of words and phrases, the details are complex, and vary sharply from language to language. For example, expressions for days of the week and months of the year are capitalized in English, but not in Spanish, French, Swedish, or Finnish, though they may be understood as proper names in all of these. Languages differ in whether most elements of multiword proper names are capitalized (American English has House of Representatives, in which lexical words are capitalized) or only the initial element (as in Slovenian Državni zbor, "National Assembly"). In Czech, multiword settlement names are capitalized throughout, but non-settlement names are only capitalized in the initial element, though with many exceptions.
In modern English orthography, it is the norm for recognized proper names to be capitalized. The few clear exceptions include summer and winter (contrast April and Easter). It is also standard that most capitalizing of common nouns is considered incorrect, except of course when the capitalization is simply a matter of text styling, as at the start of a sentence or in titles and other headings. See Letter case § Title case.
Although these rules have been standardized, there are enough gray areas that it can often be unclear both whether an item qualifies as a proper name and whether it should be capitalized: "the Cuban missile crisis" is often capitalized ("Cuban Missile Crisis") and often not, regardless of its syntactic status or its function in discourse. Most style guides give decisive recommendations on capitalization, but not all of them go into detail on how to decide in these gray areas if words are proper nouns or not and should be capitalized or not.[d]
Words or phrases that are neither proper nouns nor derived from proper nouns are often capitalized in present-day English: Dr, Baptist, Congregationalism, His and He in reference to the Abrahamic deity ("God"). For some such words, capitalization is optional or dependent on context: northerner or Northerner; aboriginal trees but Aboriginal land rights in Australia. When the comes at the start of a proper name, as in the White House, it is not normally capitalized unless it is a formal part of a title (of a book, film, or other artistic creation, as in The Keys to the Kingdom).
Nouns and noun phrases that are not proper may be uniformly capitalized to indicate that they are definitive and regimented in their application (compare brand names, discussed earlier). For example, Mountain Bluebird does not identify a unique individual, and it is not a proper name but a so-called common name (somewhat misleadingly, because this is not intended as a contrast with the term proper name). Such capitalization indicates that the term is a conventional designation for exactly that species (Sialia currucoides), not for just any bluebird that happens to live in the mountains.[e]
Words or phrases derived from proper names are generally capitalized, even when they are not themselves proper names. For example, Londoner is capitalized because it derives from the proper name London, but it is not itself a proper name (it can be limited: the Londoner, some Londoners). Similarly, African, Africanize, and Africanism are not proper names, but are capitalized because Africa is a proper name. Adjectives, verbs, adverbs, and derived common nouns that are capitalized (Swiss in Swiss cheese; Anglicize; Calvinistically; Petrarchism) are sometimes loosely called proper adjectives (and so on), but not in mainstream linguistics. Which of these items are capitalized may be merely conventional. Abrahamic, Buddhist, Hollywoodize, Freudianism, and Reagonomics are capitalized; quixotic, bowdlerize, mesmerism, and pasteurization are not; aeolian and alpinism may be capitalized or not.
Some words or some homonyms (depending on how a body of study defines "word") have one meaning when capitalized and another when not. Sometimes the capitalized variant is a proper noun (the Moon; dedicated to God; Smith's apprentice) and the other variant is not (the third moon of Saturn; a Greek god; the smith's apprentice). Sometimes neither is a proper noun (a swede in the soup; a Swede who came to see me). Such words that vary according to case are sometimes called capitonyms (although only rarely: this term is scarcely used in linguistic theory and does not appear in the Oxford English Dictionary).
In past centuries, orthographic practices in English varied widely. Capitalization was much less standardized than today. Documents from the 18th century show some writers capitalizing all nouns, and others capitalizing certain nouns based on varying ideas of their importance in the discussion.
Historical documents from the early United States show some examples of this process: the end (but not the beginning) of the Declaration of Independence (1776) and all of the Constitution (1787) show nearly all nouns capitalized; the Bill of Rights (1789) capitalizes a few common nouns but not most of them; and the Thirteenth Constitutional Amendment (1865) capitalizes only proper nouns.
In most alphabetic languages brand names and other commercial terms that are nouns or noun phrases are capitalized whether or not they count as proper names. Not all brand names are proper names, and not all proper names are brand names.
In non-alphabetic scripts proper names are sometimes marked by other means. In Egyptian hieroglyphs, parts of a royal name were enclosed in a cartouche: an oval with a line at one end. In Chinese script, a proper name mark (a kind of underline) has sometimes been used to indicate a proper name. In the standard Pinyin system of romanization for Mandarin Chinese, capitalization is used to mark proper names, with some complexities because of different Chinese classifications of nominal types,[g] and even different notions of such broad categories as word and phrase.
European alphabetic scripts only developed a distinction between upper case and lower case in medieval times so in the alphabetic scripts of ancient Greek and Latin proper names were not systematically marked. They are marked with modern capitalization, however, in many modern editions of ancient texts. Sanskrit and other languages written in the Devanagari script, along with many other languages using alphabetic or syllabic scripts, do not distinguish upper and lower case and do not mark proper names systematically.
There also appear to be differences in language acquisition. Although Japanese does not distinguish overtly between common and proper nouns, two-year-old children learning Japanese distinguished between names for categories of object (equivalent to common names) and names of individuals (equivalent to proper names): When a previously unknown label was applied to an unfamiliar object, the children assumed that the label designated the class of object (i.e. they treated the label as the common name of that object), regardless of whether the object was inanimate or not. However, if the object already had an established name, there was a difference between inanimate objects and animals:
In English, children employ different strategies depending on the type of referent but also rely on syntactic cues, such as the presence or absence of the determiner "the" to differentiate between common and proper nouns when first learned.