Declassification refers to the act of making information that was previously classified available to the public. The opposite of declassification would be classification, which means to make information secret or confidential. The term classification is often used in reference to national security documents or files that contain sensitive information. Some other antonyms for declassification could include secrecy, concealment, hiding, and suppression. These terms indicate the presence of secrecy and that the information is not meant to be shared with others. It is essential to note that the use of antonyms for declassification depends on the context in which the word is being used.