The Demilitarized Zone (or DMZ) was an area of space established as a buffer zone between the United Federation of Planets and the Cardassian Union in 2370. Neither side was permitted to deploy armed starships or establish military bases in the Zone. Also, the border between the two powers was redrawn, resulting in some Federation colonies being turned over to Cardassian control, and vice-versa. The colonists on the affected planets were given the option of resettlement, though most refused. (TNG episode: "Journey's End")
Many civilian colonists objected to the transfer of control and secretly armed themselves, violating the terms that established the DMZ. In response, the Cardassians began secretly shipping weapons to their colonists in the DMZ to oust the Federation colonists. In response, the Federation colonists banded together as the Maquis to free themselves from Cardassian control. Many Federation citizens, including Starfleet officers, sympathized with the Maquis cause and secretly provided aid. (DS9 episodes: "The Maquis, Part I" and "The Maquis, Part II", and TNG episode: "Preemptive Strike")
The Cardassian and Federation colonists continued to fight each other until 2373, when the Cardassian Union joined the Dominion. (DS9 episode: "By Inferno's Light") The Jem'Hadar quickly overran the Demilitarized Zone, wiping out the Maquis. (DS9 episode: "Blaze of Glory")
The Jenatris Corridor was partly located within the Demilitarized Zone.