"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.
Entries linking to arcanum
arcana n.
"hidden things, mysteries," 1590s, a direct adoption of the Latin plural of arcanum "a secret, a mystery," an important word in alchemy, from neuter of adjective arcanus "secret, hidden, private, concealed" (see arcane). It was occasionally mistaken for a singular and pluralized as arcanas, because arcana is far more common than arcanum.