- You may also be looking for the TNG episode, "The Neutral Zone".
A neutral zone was an area of space not claimed by any political power. Often, these areas were created as buffer zones and borders between spacefaring civilizations. In this sense, a neutral zone is somewhat similar to a Demilitarized Zone, such as the demilitarized zone that existed between the Cardassian Union and the United Federation of Planets was an example of this. (TNG: "Journey's End")
One prominent example of a neutral zone was the Romulan Neutral Zone between the United Federation of Planets and the Romulan Empire. A neutral zone also existed with the Klingon Empire in 2285. This neutral zone was violated by Commander Kruge. (TOS: "Balance of Terror", Star Trek III: The Search for Spock)
A similated neutral zone, appearing as a small oval area of space, was defended by Klingon cruisers, was a part of the simulated Kobayashi Maru scenario. (Star Trek II: The Wrath of Khan)
When Captain Hikaru Sulu offered assistance to the Klingon High Command following the destruction of Praxis, he was told by Brigadier General Kerla that their assistance was not required and to "obey treaty stipulations and remain outside the Neutral Zone." The neutral zone was again mentioned during the beginning of negotiations between the Klingon Empire and the Federation which led to the Khitomer Accords, when the Klingon Chancellor Gorkon proposed removing starbases from the neutral zone. (Star Trek VI: The Undiscovered Country)
Background
The name Klingon Neutral Zone was never mentioned on-screen. The nature of the "neutral zone" mentioned in contexts involving Klingons is unclear. Many speculate that this zone was created by the Treaty of Organia, and eliminated by the Khitomer Accords.
The only "real" neutral zones that have ever existed in the world were the Saudi-Iraqi neutral zone and the Saudi-Kuwaiti neutral zone, both established in the 1920s and disbanded by the 1990s. It is unknown if Star Trek writers based the Trek neutral zones on these two bodies or came up with the term independently.