The Treaty of Algeron was a legal document to which at least two interstellar political entities were signatories.
The treaty was ratified by the United Federation of Planets and the Romulan Star Empire after the Tomed Incident in 2311. Article IV of the treaty proscribed the development of cloaking device technology by the Federation. Future senator Merken tr'Vreenak was an aide to the Romulan delegation. (TNG: "The Defector"; TLE novel: Serpents Among the Ruins; Bait and Switch: "Flaihhsam s'Spahkh")
A special dispensation was given to the escort USS Defiant, in the 2370s, to have use of a cloaking device. This was negotiated by Ambassador Spock in 2371, shortly after the loss of the USS Odyssey in the first contact between the Federation and the Dominion. This required a Romulan Senate vote to make an exception to Article IV, which passed 507 to 494 with seven abstentions and fourteen senators absent. (DS9: "The Search, Part 1", "The Search, Part 2", "Visionary"; Bait and Switch: "Flaihhsam s'Spahkh")