Wiktionary
da-notice
n. (context UK English) A defence advisory notice, an official (but unenforceable) request to news editors not to publish or broadcast items on specified subjects for reasons of national security.
Wikipedia
DA-Notice
A DA-Notice (Defence Advisory Notice)—called a Defence Notice (D-Notice) until 1993—is an official request to news editors not to publish or broadcast items on specified subjects for reasons of national security. The system is still in use in the United Kingdom.