A perimeter grid was a type of sensor net used to detect starships that violated an area of space.
The highly territorial Swarm species protected their space with a form of perimeter grid that utilized a series of interlaced tachyon beams to detect encroaching ships. In 2373, the crew of the USS Voyager devised a method of entering the grid without being detected by modifying their shields to refract the beams around the ship. However Voyager later encountered a Swarm ship that emitted a polaron pulse disrupting the new shield configuration, making the ship visible to the grid and therefore other Swarm vessels. (VOY: "The Swarm")
Until at least 2374, the xenophobic B'omar maintainted a perimeter grid to detect unauthorised vessels entering their space. The grid was sensitive enough to even detect and track small objects in space such as stellar particles. (VOY: "The Raven")