The Collaborative International Dictionary of English v.0.48:

Arcanum \Ar*ca"num\, n.; pl. Arcana. [L., fr. arcanus closed,
   secret, fr. arca chest, box, fr. arcere to inclose. See
   Ark.]
   1. A secret; a mystery; -- generally used in the plural.
      [1913 Webster]

            Inquiries into the arcana of the Godhead.
                                                  --Warburton.
      [1913 Webster]

   2. (Med.) A secret remedy; an elixir. --Dunglison.
      [1913 Webster]

WordNet (r) 3.0 (2006):

arcanum
    n 1: information known only to a special group; "the secret of
         Cajun cooking" [syn: secret, arcanum]