SynonymDefinitionAntonym.com - English Synonym Dictionary

Something that baffles understanding and cannot be explained

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

Information known only to a special group

The next to highest level of official classification for documents

Not open or public; kept private or not revealed

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

Not expressed

(of information) given in confidence or in secret

Not openly made known

Conducted with or marked by hidden aims or methods

Designed to elude detection

Indulging only covertly

Hidden from general view or use

Communicated covertly