In the mirror universe, the Terran Neutral Zone was a ten light year wide zone around the Galactic Commonwealth. It was established in the year 2377 by the Tripartite Armistice and open to expansion to the Commonwealth. (ST novel: Rise Like Lions)
In the armistice signed between the then-Terran Protectorate, the Klingon Empire and the Cardassian Union in 2377, more than two dozen worlds were formally ceded to the Commonwealth in a 20 light year zone around Sol. The entire territory was surrounded by the Terran Neutral Zone. The Commonwealth was intending on peaceful expansion by voluntary admission of worlds. The territory shared borders with the Klingon Empire, Cardassian Union, Romulan Star Empire and the Taurus Pact.