The Great Peace, known in Klingon as Roj tIn, is an event prophesied by the Organians.

This period in galactic history was predicted by the Organian race when they imposed a peace between the United Federation of Planets and the Klingon Empire. The transcended race believed that, in time, the two empires would become friends and allies in the future.

During the tense period of cold war between the Empire and the Federation, the Klingons orchestrated an elaborate ruse where they tricked James T. Kirk into believing he had been accidently sent a hundred years into the future where the Great Peace had occured thanks to the New Klingons. This was a plot in order to allow the Klingons to sneak a fleet to Earth and destroy the leadership of the Federation but it failed. (TOS novel: Timetrap)