secret

not openly made known

“a secret marriage”

communicated covertly

“their secret signal was a wink”

indulging only covertly

“a secret alcoholic”

not open or public; kept private or not revealed

“a secret formula”

something that should remain hidden from others (especially information that is not to be passed on)

“the combination to the safe was a secret”

the next to highest level of official classification for documents

conducted with or marked by hidden aims or methods

“secret missions”

hidden from general view or use

“a secret garden”

not expressed

“secret (or private) thoughts”

given in confidence or in secret

“their secret communications”

having an import not apparent to the senses nor obvious to the intelligence; beyond ordinary understanding

“the secret learning of the ancients”