Mysterious
Part of speech: adjective
Definition: Of unknown origin Having unknown qualities
Example sentence: There is a mysterious cycle in human events. To some generations much is given. Of other generations much is expected. This generation of Americans has a rendezvous with destiny.
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 of life is enjoying the passage of time.