A D-notice was an act that could be issued by the government or military. It was issued in cases of national security to keep an investigation from the press or to shut down an investigation.
Greg Sutton mentioned to Petra Williams before he went to be interviewed by James Stevens about the Inferno Project that "We don't pay much attention to pommie D-notices where I come from", and figured Stevens wouldn't believe a word of the story anyway. (PROSE: Who Killed Kennedy)
Sarah Jane Smith and the Third Doctor were investigating the strange events behind Isaac's death when Sarah's editor received a D-notice. The Brigadier and the Doctor tried to convince the higher powers to rescind the D-notice, but gave in when they learned it had been issued by a future version of the Doctor. (PROSE: An Overture Too Early)