A defense net was a coordinated number of assets protecting a certain location or region of space.
A defense net, which consisted of artificial satellites, was orbiting a penal moon in the Gamma Quadrant. The net forced the USS Yangtzee Kiang to crash on the moon in 2369. Golin Shel-la believed the defense net to be near impregnable, but Benjamin Sisko assured him a Starfleet rescue team would eventually penetrate it and rescue them. (DS9: "Battle Lines")
The territory of the Kazon-Nistrim was protected by a defense net. In 2372, Teirna 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")