In Star Trek: The Next Generation, what device lets a starship detect ships hidden by a cloaking device?

In Star Trek: The Next Generation, a tachyon detection grid lets a starship detect ships hidden by a cloaking device.

The tachyon detection grid is a sensor system designed to reveal the disturbances created by a cloaked vessel. Tachyons are fictional faster-than-light particles used throughout Star Trek as a way to detect changes in subspace and locate otherwise hidden objects. By deploying a broad grid of sensors, a fleet can search a region rather than relying on one ship’s ordinary scanners.

The technology is particularly associated with the Federation’s response to the Romulan threat. Cloaking devices conceal a ship from normal visual and sensor detection, but they are not always perfect. Detection systems can exploit energy emissions, gravitational effects, or subspace disturbances that the cloak cannot completely hide.

The grid is not the same as a cloaking device. A cloak hides a vessel; the tachyon grid is a countermeasure intended to expose it.