noun

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The part of speech that is used to name a person, place, thing, quality, or action and can function as the subject or object of a verb, the object of a preposition, or an appositive.
2. n. Any of the words belonging to this part of speech, such as neighbor, window, happiness, or negotiation.

Century Dictionary and Cyclopedia

1. n. In grammar, a name; a word that denotes a thing, material or immaterial; a part of speech that admits of being used as subject or object of a verb, or of being governed by a preposition. Any part of speech, or phrase, or clause thus used is a noun, or the equivalent of a noun, or used as a noun: thus, he is prodigal of ifs and buts; fare well is a mournful sound; that he is gone is true enough. Nouns are called proper, common, collective, abstract, etc. (See these words.) The older usage, and less commonly the later, make the word noun include both the noun and the adjective, distinguishing the former as noun substantive and the latter as noun adjective. Abbreviated n.

Wiktionary

1. n. grammar A word that can be used to refer to a person, animal, place, thing, phenomenon, substance, quality, or idea; one of the basic parts of speech in many languages, including English.
2. v. transitive To convert a word to a noun.

GNU Webster's 1913

1. n. (Gram.) A word used as the designation or appellation of a creature or thing, existing in fact or in thought; a substantive.

WordNet 3.0

1. n. a content word that can be used to refer to a person, place, thing, quality, or action
2. n. the word class that can serve as the subject or object of a verb, the object of a preposition, or in apposition