| secret | (adjective) the next to highest level of official classification for documents | - |
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| (adjective) not open or public; kept private or not revealed | - |
| (adjective) having an import not apparent to the senses nor obvious to the intelligence; beyond ordinary understanding | Synonyms: mysterious, mystic, mystical, occult, orphic |
| (adjective) not expressed | Synonyms: private |
| (adjective) (of information) given in confidence or in secret | Synonyms: confidential |
| (adjective) not openly made known | Synonyms: unavowed |
| (adjective) conducted with or marked by hidden aims or methods | Synonyms: clandestine, cloak-and-dagger, hole-and-corner, hugger-mugger, hush-hush, surreptitious, undercover, underground |
| (adjective) designed to elude detection | Synonyms: hidden |
| (adjective) indulging only covertly | - |
| (adjective) hidden from general view or use | Synonyms: privy, secluded |
| (adjective) communicated covertly | - |
| (noun) something that baffles understanding and cannot be explained | Synonyms: closed book, enigma, mystery |
| (noun) something that should remain hidden from others (especially information that is not to be passed on) | - |
| (noun) information known only to a special group | Synonyms: arcanum |