Confidential
Part of speech: adjective
Definition: (meant to be) kept secret within a certain circle of persons; not intended to be known publicly
Example sentence: Of all those in the army close to the commander none is more intimate than the secret agent; of all rewards none more liberal than those given to secret agents; of all matters none is more confidential than those relating to secret operations.
Secret
Part of speech: noun
Definition: Knowledge that is hidden and intended to be kept hidden.
Part of speech: verb
Definition: To make or keep secret.
Part of speech: adjective
Definition: Being or kept hidden.
Example sentence: The secret to success is to offend the greatest number of people.