The Starfleet order that forbade any vessel from approaching Talos IV, carrying the only death penalty left in Federation law, which Spock defied to take Christopher Pike back there.
General Order 7 was issued after the Enterprise's first visit to Talos IV under Christopher Pike. Starfleet judged the illusion-casting power of the Talosians so dangerous that no ship was to go near the planet for any reason, and breaking the order was the one offence still punishable by death.
The order drives the two-part story The Menagerie. In 2267 Spock abducts the crippled Pike, takes control of the Enterprise and sets course for Talos IV, then submits to a court-martial at which he shows the events of the earlier mission. At the end Starfleet suspends the order for this one occasion and Pike is allowed to live out his life among the Talosians.
The quarantine is still in force when Discovery travels to Talos IV with Spock and Michael Burnham in If Memory Serves.