Nouns are a part of speech, classified into proper nouns (e.g. "Janet"), common nouns (e.g. "girl"), collective nouns (e.g. "bunch", "herd") and pronouns (e.g. "she" and "which"). Each noun is a word representing a thing, in a sense that probably cannot be
defined rigorously, but is broad enough to include organisms, places, qualities, and actions, both actual and imagined.
Further classifications include the distinction between concrete nouns and abstract nouns. Concrete nouns refer to definite object (e.g. chair, apple, Janet) and abstract nouns refer to ideas or concepts (e.g. justice, liberty). While sometimes useful, the boundaries between these two are not always clear.
In sentences, nouns occur in several different ways, the most common being as subjects (performers of action), or objects (recipients of action). In the sentence "John wrote me a letter", "John" is a subject; "me" and "letter" are objects.
Proper nouns are capitalized in English and most or all other languages that use the Latin alphabet; this is one easy way to recognize them. (This fails, however, in German, in which nouns of all types are capitalized.) Other words that are often or always capitalized in English include:
trademarks (e.g. "Dumpster" and "Kleenex") and
words derived from proper nouns (e.g. "Aristotelian" and "Canadian", but "chauvinism" and "quixotic").
This "proper non-noun" phenomenon of English is by no means a universal trait of languages: it does not occur in Romance languages, nor, despite their common
Germanic roots, in German. (Another capitalization anomaly in English is the word "I"; it could logically be construed as a proper name referring to a unique object, even though it is a pronoun normally used by anyone who speaks of themself.)
Sometimes the same word can appear as both a common noun and a proper noun, where one such entity is special; for example:
there can be many gods, but there is only one God.
there can be many internets (networks of TCP/IP networks), but the largest internet in the world is the Internet.
A mass noun is a type of common noun that represents a substance not easily quantified by a number. Mass nouns do not require limiting modifiers ("an", "two", "several", "many", etc.) and are not normally pluralized. Examples from English include "cheese", "laughter", and "precision".
