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]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "archaism":
   Pre-Raphaelitism, ancient manuscript, antiquarianism, antique,
   antiquity, archaeology, archaicism, artifact, cave painting,
   classicism, eolith, fossil, medievalism, mezzolith, microlith,
   neolith, obsolete, obsoletism, paleolith, petrification,
   petrified forest, petrified wood, petroglyph, plateaulith, relic,
   reliquiae, remains, ruin, ruins, survival, vestige