A Starfleet emergency communication of the highest secrecy, meant for a captain's eyes only and leaving no computer record, used by Walker Keel to warn Picard.
Code 47 is the designation of a Starfleet emergency transmission so secret that it is delivered to the captain alone and no record of it is kept in the ship's computer. Even discussing its existence with the crew is discouraged.
It appears in Conspiracy. A Code 47 message wakes Picard in his quarters, and he finds his old friend Captain Walker Keel on the screen. Keel will not explain himself over the channel and asks for a secret meeting on the abandoned mining world Dytallix B.
There Keel and two other captains tell Picard that something is subverting Starfleet Command. Keel's ship, the Horatio, is destroyed soon afterwards, and Picard takes the Enterprise to Earth, where he and Riker uncover the parasites that have taken over senior officers.