If official information is declassified, it used to be secret — stamped, locked, only for certain people — and now the government (or the army, or the agency) has taken that secret status off. The papers, photos, or files can be shown, published, or requested in a way they could not before. People say “declassified documents,” “a declassified report,” “the files were declassified after thirty years.” It is almost always about state or institutional secrets, not about your diary or a company’s ordinary files (those are more often confidential, then released). Though actually, news stories love the word, so you will also hear it a little loosely for anything that “comes out of the dark.” Strictly, declassified means it was classified first. That is different from unclassified, which can mean it was never secret to begin with. The adjective is the -ed form of the verb declassify.