The Treaty of Organia, also known as the Organian Peace Treaty, was an agreement enforced by the powerful noncorporeal beings of Organia upon the United Federation of Planets and the Klingon Empire, ending the Federation-Klingon War in 2267. (TOS: "Errand of Mercy")
According to the terms of the treaty, each side could lay claim to a disputed planet inside the Neutral Zone between the two powers, and was given the opportunity to demonstrate its ability to develop that planet most efficiently. Also, each side was granted the right to demand use of the other's shore leave facilities along the border. (TOS: "The Trouble with Tribbles")
The Treaty of Organia was eventually superseded by the Khitomer Accords of 2293, which bore out the Organians' prediction that the Klingons and the Federation would one day become friends. (Star Trek VI: The Undiscovered Country)
According to the novel The Klingon Gambit, by Robert E. Vardeman, the Treaty of Organia covered a seven-hundred-and-fifty parsec zone. The novel How Much for Just the Planet? sums up the Organian Treaty as "Don't get grabby, or you'll get your fingers burned."