In English grammar, a proper noun is a noun having certain grammatical properties. In particular, proper nouns cannot normally be used with an article (as in *“a France” or *“the India”) or other determiner (as in *“that England” or even *“much Senegal”, though “this England” and “much of Senegal” are fine), and are not normally found in the plural (as in *“Germanys” or *“Australias”). A proper noun usually denotes a single unique entity, but the term proper noun pertains to a word's grammatical properties rather than to its meaning.
"John Albert Magnus Smith" is an archetypal proper noun because it is almost always intended to refer to a specific individual. Of course, there probably have been a few persons with that exact name. Most telephone books in the English-speaking world have several "JA Smiths". "Smith", "John", "Johnny", "Crazylegs", "Newbie", "Bozo", and "that guy with the horrible green tie over by the phone there" might all be names intended to uniquely the same particular individual, as would "John A. Smith of Cedarhurst, NY.", "John Smith, US social security number 234-56-7890", or "Mr. Smith, the owner of the red Corvette formerly parked in the loading zone". All of these terms can serve as proper nouns grammatically.
In English orthography, proper nouns are usually capitalized, as are many adjectives and common nouns that derive from proper nouns; for example, England (proper noun), English (adjective), and Englishman (common noun, plural Englishmen) are all capitalized. However, not all capitalized words come from proper nouns; for example, U.S. legal works sometimes capitalize the common noun state, and conversely, a number of modern corporations have chosen to brand themselves with lowercase first letters.
A number of proper nouns meet the above description, except that they are normally used together with the rather than standing alone; hence, for example, one normally speaks of the Atlantic Ocean, not of *Atlantic Oceans nor of *an Atlantic Ocean. Some proper nouns are used both ways; for example, Sudan and the Sudan are synonymous.
Further, when a proper noun is preceded by a non-restrictive adjective, it frequently takes the (as in “the great Gatsby” or “the talented Mr. Ripley”). This resembles the use of an epithet, as in “Ivan the Terrible”.
A number of proper nouns, such as the names of many mountain ranges and island chains, are inherently plural; hence one speaks of the Alps, the Bahamas, and so on, but does not normally describe an individual mountain as *an Alp or an individual island as *a Bahama.
Moreover certain group nouns the Nine Riders, the Founding Fathers are capitalised, as they are the name of the group, in some cases the singular can be back-formed "He was a Founding Father."
Although proper nouns have certain typical features that distinguish them from common nouns, most can also be used as common nouns with certain meanings. For example, given names, such as Jim, are normally proper nouns; one asks “Do you know Jim?”, not *“Do you know the Jim?”. However, one can also use Jim as a common noun meaning “a person named ‘Jim’”; hence, one asks “Do you know the Jim who works on the third floor?” (=“Do you know the man named ‘Jim’ who works on the third floor?”). The same is true of surnames, which as common nouns are frequently found in the plural, as in the phrase keeping up with the Joneses. Even proper nouns denoting a single entity entity can display this behavior if (for example) the entity can be seen in different ways, or if it changes over time. Hence one might say, for example, “The Europe of 1600 was very different from the Europe we know today.” Interestingly, English’s normal spelling patterns for plurals are frequently subverted with proper nouns; for example, two women named Mary are together Marys, not usually Maries.
This is analogous to the use of non-count nouns as count nouns — for example, one cannot ordinarily speak of *two rices, but one can say, “white rice and long-grain rice are two rices grown in the U.S.” — and of count nouns as non-count nouns — for example, one cannot ordinarily say *a bit of headphone, but one can say, “In his rage, he completely destroyed the headphones, and afterwards there were little bits of headphone scattered across the floor.”
As a further complication, proper nouns or nouns not readily distinguished from proper nouns can be used as non-count nouns, as in: "The living room was too much Mary for Ted to feel comfortable in it."
Many common nouns may be used as proper nouns; for example, mother is a common noun, as in “His mother just turned sixty-five”, but may be used as a proper noun, as in “Have you seen Mother today?”. As in this example, common nouns are traditionally capitalized when used as proper nouns.
Many nouns can be used as epithets to uniquely identify individuals within contexts. "Meathead" was used in a well-known US television show to uniquely identify the main character's son-in-law.
English treats as proper nouns the names of various unique entities designated by scientific bodies. Examples include taxonomic names, names of celestial objects and features of celestial objects.
Proper nouns, like other nouns, may be used as attributive modifiers, as in “New York minute”, “U.S. government”, and the like. In this use they are called attributive nouns or noun adjuncts, just as with common nouns.
"Proper noun" is one of the headings that English Wiktionary uses to categorize and describe definitions in entries. Because Wiktionary has very strict criteria of inclusion that normally prevent adding English proper nouns such as personal and corporate names, the mix of types of proper nouns found in Wiktionary differs from that found in ordinary speech or writing.