Source : Webster's Revised Unabridged Dictionary (1913)

Archaism \Ar"cha*ism\, n. [Gr. 'archai:smo`s, fr. 'archai^os
   ancient, fr. 'archh` beginning: cf. F. archa["i]sme. See
   {Arch}, a.]
   1. An ancient, antiquated, or old-fashioned, word,
      expression, or idiom; a word or form of speech no longer
      in common use.

   2. Antiquity of style or use; obsoleteness.

            A select vocabulary corresponding (in point of
            archaism and remoteness from ordinary use) to our
            Scriptural vocabulary.                --De Quincey.

Source : WordNet®

archaism
     n : the use of an archaic expression [syn: {archaicism}]