The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. Antiquity of style or use; obsoleteness.
      [1913 Webster]

            A select vocabulary corresponding (in point of
            archaism and remoteness from ordinary use) to our
            Scriptural vocabulary.                --De Quincey.
      [1913 Webster]

WordNet (r) 3.0 (2006):

archaism
    n 1: the use of an archaic expression [syn: archaism,
         archaicism]