se•cret

– adj.
1. done, made, or conducted without the knowledge of others.
2. kept from the knowledge of any but the initiated or privileged.
3. faithful or cautious in keeping confidential matters confidential; close-mouthed; reticent.
4. designed or working to escape notice, knowledge, or observation.
5. secluded, sheltered, or withdrawn.
6. beyond ordinary human understanding; esoteric.
7. a. bearing the classification secret.
b. limited to persons authorized to use information documents, etc., so classified.
– noun
8. something that is or is kept secret, hidden, or concealed.
9. a mystery: the secrets of nature.
10. a reason or explanation not immediately or generally apparent.
11. a method, formula, plan, etc., known only to the initiated or the few: the secret of happiness; a trade secret.
12. a classification assigned to information, a document, etc., considered less vital to security than top-secret but more vital than confidential, and limiting its use to persons who have been cleared, as by various government agencies, as trustworthy to handle such material.
- idiom
13. unknown to others; in private.

mys•ter•y

– noun
1. anything that is kept secret or remains unexplained or unknown.
2. any affair, thing, or person that presents features or qualities so obscure as to arouse curiosity or speculation.
3. a novel, short story, play, or film whose plot involves a crime or other event that remains puzzlingly unsettled until the very end.
4. obscure, puzzling, or mysterious quality or character.
5. any truth that is unknowable except by divine revelation.

ob•scure

– adj.
1. not clear or plain; ambiguous, vague, or uncertain.
2. not clear to the understanding; hard to perceive.
3. not expressing the meaning clearly or plainly.
4. indistinct to the sight or any other sense; not readily seen, heard, etc.; faint.
5. inconspicuous or unnoticeable.
6. of little or no prominence, note, fame, or distinction.
7. far from public notice, worldly affairs, or important activities; remote; retired.
8. lacking in light or illumination; dark; dim; murky.
9. enveloped in, concealed by, or frequenting darkness.
10. not bright or lustrous; dull or darkish, as color or appearance.
– verb
11. to conceal or conceal by confusing.
12. to make dark, dim, indistinct, etc.