The buffer zone between the Federation and Romulans.
The Neutral Zone is a region of space that serves as a buffer between the Federation and the Romulan Star Empire, established by a treaty that followed a war between the two powers. It is a border that must not be crossed by either side, and it is monitored by outposts and patrols.
The Zone was created after the Earth-Romulan War, and the treaty marks the beginning of a long period of tension between the two powers. For many years, there was little contact between them, and the Romulans remained a mystery to the Federation. Because it is such a sensitive area, any incident that takes place in it can easily lead to war.
It appears in several episodes of the original series, including Balance of Terror, in which a Romulan warship crosses the line, and in the Next Generation episode The Neutral Zone, which marks the return of the Romulans after years of silence. The border is the scene of many incidents and is a central element of the Federation's political situation.