A tachyon detection grid is a method of detecting cloaked starships. Originally developed by Starfleet it involves various starships or sentry posts transmitting tachyon beams between them to create a tachyon field. Any cloaked vessel attempting to navigate through the field would be exposed.
The first practical application was by a Starfleet task force led by the USS Enterprise-D in 2368. The grid successfully managed to detect a Romulan convoy as it attempted to penetrate Klingon space to assist the House of Duras during the Klingon Civil War. (TNG: "Redemption II")