Classified means officially kept secret by a government or organization because the information could cause harm if revealed to the wrong people. When military plans are classified, only authorized personnel can see them. When government scientists work on classified research, they can't discuss the details with their families or friends.

The U.S. government uses levels like “confidential,” “secret,” and “top secret” to mark how carefully information must be protected. A classified document might contain details about military operations, intelligence sources, or national security matters that could endanger people or compromise important work if enemies learned about them.

In everyday conversation, people sometimes use classified more loosely to mean any secret information. A student might jokingly call their diary entries “classified” or declare that weekend plans are “classified information.” But when used seriously, classified specifically refers to official secrets that governments or organizations designate as restricted, with real consequences for unauthorized disclosure.

The opposite of classified is declassified, which means secret information that's been officially approved for public release, usually after enough time has passed that revealing it no longer poses a risk.