secret

something 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"

information known only to a special group; "the secret of Cajun cooking"

something that baffles understanding and cannot be explained; "how it got out is a mystery"; "it remains one of nature's secrets"

not open or public; kept private or not revealed; "a secret formula"; "secret ingredients"; "secret talks"

conducted with or marked by hidden aims or methods; "clandestine intelligence operations"; "cloak-and-dagger activities behind enemy lines"; "hole-and-corner intrigue"; "secret missions"; "a secret agent"; "secret sales of arms"; "surreptitious mobilization of troops"; "an undercover investigation"; "underground resistance"

not openly made known; "a secret marriage"; "a secret bride"

designed to elude detection; "a hidden room or place of concealment such as a priest hole"; "a secret passage"; "the secret compartment in the desk"

having 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"