A defense net is a coordinated series of defense systems covering interstellar distances.
The territory of the Kazon-Nistrim is protected by a defense net. In 2372, Tierna gave the USS Voyager Maje Culluh's command codes, allowing them to plot a course avoiding the Nistrim defense net. However, Tierna was in fact sending them into a trap. (VOY: "Basics, Part I")