2.adjectiveFormally 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."
3.adjectiveNot meant to be disclosed by a person or organization.
4.verbTo identify by or divide into classes; to categorize
"Should we classify "make up" as an idiom or as a phrasal verb?"
5.verbTo declare something a secret, especially a government secret