Classified

classify + -ed

Verb

1. classified

   (past of classify)
   Make sure all the field observations are classified by species.

Adjective

1. Formally assigned by a government to one of several levels of sensitivity, usually (in English) top secret, secret, confidential, and, in some countries, restricted; thereby making disclosure to unauthorized persons illegal.We do not discuss specific interrogation techniques because they are classified information.
2. Not meant to be disclosed by a person or organization.I won't say who I'm going to the prom with; that's classified.

From classified advertisement, by shortening.

Full definition of classified

Noun

1. (informal) A classified advertisement in a newspaper or magazine.He ran a classified for the tools he wanted to sell.