(n)secretsomething that should remain hidden from others (especially information that is not to be passed on)“the combination to the safe was a secret”, “he tried to keep his drinking a secret”
(n)mystery, enigma, secret, closed booksomething that baffles understanding and cannot be explained“how it got out is a mystery”, “it remains one of nature's secrets”
(s)hidden, secretdesigned to elude detection“a hidden room or place of concealment such as a priest hole”, “a secret passage”, “the secret compartment in the desk”
(s)mysterious, mystic, mystical, occult, secret, orphichaving an import not apparent to the senses nor obvious to the intelligence; beyond ordinary understanding“mysterious symbols”, “the mystical style of Blake”, “occult lore”, “the secret learning of the ancients”