What is some synonyms for the word secret?

Here are some synonyms for the word "secret," categorized by their nuance:

General synonyms:

* Confidential

* Private

* Hidden

* Concealed

* Classified

* Hush-hush

* Top secret

* Undisclosed

* Unknown

Synonyms emphasizing secrecy:

* Covert

* Clandestine

* Stealthy

* Furtive

* Under wraps

Synonyms emphasizing the nature of the information:

* Mystery

* Enigma

* Riddle

* Clue

* Insider information

* Trade secret

Synonyms emphasizing the act of keeping something secret:

* Concealment

* Suppression

* Withholding

* Keeping quiet

Synonyms emphasizing the result of keeping something secret:

* Unknown

* Unsolved

* Unrevealed

* Uncovered

The best synonym to use will depend on the specific context and the shade of meaning you want to convey.