arcana (n.)

"hidden, secret," 1540s, from Latin arcanus "secret, hidden, private, concealed," from arcere "to close up, enclose, contain," from arca "chest, box, place for safe-keeping," from PIE root *ark- "to hold, contain, guard" (source also of Greek arkos "defense," arkein "to ward off;" Armenian argel "obstacle;" Lithuanian raktas "key," rakinti "to shut, lock").

"a secret, a mystery," proper singular form of arcana (q.v.); in alchemy, a supposed great secret of nature, hence, generally, "the secret virtue" of anything.