1.something that should remain hidden from others (especially information that is not to be passed on);

the combination to the safe was a secret

2.information known only to a special group;

the secret of Cajun cooking

3.something that baffles understanding and cannot be explained;

how it got out is a mystery

4.not open or public; kept private or not revealed;

a secret formula

5.conducted with or marked by hidden aims or methods;

clandestine intelligence operations

6.not openly made known;

a secret marriage

7.communicated covertly;

their a secret signal was a wink

8.not expressed;

secret (or private) thoughts

9.designed to elude detection;

a hidden room or place of concealment such as a priest hole

10.hidden from general view or use;

a privy place to rest and think

11.(of information) given in confidence or in secret;

closet information

12.indulging only covertly;

a closet alcoholic

13. having an import not apparent to the senses nor obvious to the intelligence; beyond ordinary understanding;

mysterious symbols

14. the next to highest level of official classification for documents