Source : Webster's Revised Unabridged Dictionary (1913)

Adnoun \Ad"noun`\, n. [Pref. ad- + noun.] (Gram.)
   An adjective, or attribute. [R.] --Coleridge.

Source : WordNet®

adnoun
     n : an adjective used as a noun; "`meek' in `blessed are the
         meek' is an adnoun"