Secret

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

not open or public

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

the next to highest level of official classification for documents