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.
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            Inquiries into the arcana of the Godhead.
                                                  --Warburton.
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   2. (Med.) A secret remedy; an elixir. --Dunglison.
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