To declassify is the official act of taking secrecy off information that was classified. A government department declassifies files; a board reviews them; a date arrives and the stamp changes. After that, the material is no longer treated as a state secret, though some lines may still be blacked out. The verb is bureaucratic and news-like. Ordinary people do not declassify their group chats. You declassify documents, records, intelligence, reports. The opposite move is to classify (to make it secret, or to set its secret level). Unclassify is not the usual verb; English prefers declassify for this undoing. From de- + classify: classify here is the security sense (“put into a secret class”), not the school sense of sorting animals into groups — though it is the same verb underneath, and that is why the word looks like a science word at first.

解密、解除機密