secret

secret - noun

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

information known only to a special group

something that baffles understanding and cannot be explained

secret - adjective

not open or public; kept private or not revealed
conducted with or marked by hidden aims or methods
not openly made known
communicated covertly
not expressed
designed to elude detection
hidden from general view or use
(of information) given in confidence or in secret
indulging only covertly
having an import not apparent to the senses nor obvious to the intelligence; beyond ordinary understanding
the next to highest level of official classification for documents

secret - thesaurus