The word "arcanum" (pluralized as "arcana") came from Latin "arcanus," meaning "secret," and entered English as the Dark Ages gave way to the Renaissance.

What is the singular of Arcana?

​[singular] either of the two groups of cards in a tarot pack, the major arcana and the minor arcana.

How do you use Arcana in a sentence?

Even the wisest of men have hoped for a full knowledge of the arcana of metals. We unfold the hidden operations, the very arcana of Nature, whom we unclothe as it were to her very nakedness.