What does arcanum mean?
The definition of arcanum
Ar*ca”num, n.; pl. Arcana.
Etym: [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. Inquiries into the arcana of the Godhead. Warburton.
2 (Med.)
A secret remedy; an elixir. Dunglison.