Definitions of secret:

• noun:   information known only to a special group

  Example: "The secret of Cajun cooking"

• noun:   something that should remain hidden from others (especially information that is not to be passed on)

  Example: "The combination to the safe was a secret"

• noun:   something that baffles understanding and cannot be explained

  Example: "It remains one of nature's secrets"

• adjective:   communicated covertly

  Example: "Their a secret signal was a wink"

• adjective:   not open or public; kept private or not revealed

  Example: "A secret formula"

• adjective:   the next to highest level of official classification for documents
• adjective:   not expressed

  Example: "Secret (or private) thoughts"

• adjective:   indulging only covertly
• adjective:   designed to elude detection

  Example: "A secret passage"

• adjective:   not openly made known

  Example: "A secret marriage"

• adjective:   (of information) given in confidence or in secret

  Example: "Their secret communications"

• adjective:   hidden from general view or use

  Example: "A secret garden"

• adjective:   having an import not apparent to the senses nor obvious to the intelligence; beyond ordinary understanding

  Example: "The secret learning of the ancients"

• adjective:   conducted with or marked by hidden aims or methods

  Example: "Secret missions"