secret

it's a secret 

 tell somebody a secret/let somebody in on a secret also  

reveal a secret formal    

keep a secret   =not tell someone about something    

your secret is safe with me  =I will not tell anyone about it  

 big secret  =an important secret, or something that is known by very few people   

  little  secret =a secret that is not very important  

  be a closely-guarded/well-kept secret  =a secret that very few people know about  

state/official secret 

 trade secret  

dark secret  = a secret about something bad  

 somebody's  innermost secrets = someone's most personal secrets

 it's no secret (that) (=everyone knows about this 

 it's an open secret (that) = 

most people know about this, although you might expect it to be a secret