The Collaborative International Dictionary of English v.0.48:

Noun \Noun\ (noun), n. [OF. noun, nun, num, non, nom, F. nom,
   fr. L. nomen name. See Name.] (Gram.)
   A word used as the designation or appellation of a creature
   or thing, existing in fact or in thought; a substantive.
   [1913 Webster]

   Note: By some grammarians the term noun is so used as to
         include adjectives, as being descriptive; but in
         general it is limited to substantives.
         [1913 Webster]

WordNet (r) 3.0 (2006):

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