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 whole secret of existence is to have no fear. Never fear what will become of you, depend on no one. Only the moment you reject all help are you freed.
Cloak-and-dagger
Part of speech: adjective
Definition: Marked by furtive secrecy, often with a melodramatic tint or espionage involved.