secret
noun
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”
Synonymsarcanumsomething that baffles understanding and cannot be explained
“how it got out is a mystery”
“it remains one of nature's secrets”
adjective
not open or public; kept private or not revealed
“a secret formula”
“secret ingredients”
conducted with or marked by hidden aims or methods
“clandestine intelligence operations”
“cloak-and-dagger activities behind enemy lines”
not openly made known
“a secret marriage”
“a secret bride”
Synonymsunavowedcommunicated covertly
“their secret signal was a wink”
“secret messages”
not expressed
“secret (or private) thoughts”
Synonymsprivatedesigned to elude detection
“a hidden room or place of concealment such as a priest hole”
“a secret passage”
Synonymshiddenhidden from general view or use
“a privy place to rest and think”
“a secluded romantic spot”
(of information) given in confidence or in secret
“this arrangement must be kept confidential”
“their secret communications”
Synonymsconfidentialindulging only covertly
“a secret alcoholic”
having an import not apparent to the senses nor obvious to the intelligence; beyond ordinary understanding
“mysterious symbols”
“the mystical style of Blake”
the next to highest level of official classification for documents
Source: WordNet 3.1, Princeton University