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 space faring 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 by the terms of the Federation-Cardassian Treaty of 2370.
One prominent example of a neutral zone was the Romulan Neutral Zone established between the United Federation of Planets and the Romulan Star Empire in the 2160s following the end of the Earth-Romulan War. A neutral zone also existed with the Klingon Empire by 2285, referred to by the Klingons as the Federation Neutral Zone.