A gravitational-wave detector (aka GW detector, gravitational-wave observatory, GW observatory) is a device/facility to detect gravitational waves (GW detections). The current method of choice is a very large and very sensitive Michelson interferometer. It detects relative motion of objects some distance apart (mirrors) due to the modification of the scale of space inherent in the waves. Current detectors have light paths kilometers in length, and detect relative motion between the ends of the light paths smaller than the width of an atomic nucleus. Given movements this small, having multiple separated detectors offers a huge advantage of distinguishing local movements (e.g., a truck driving by) from the type that GWs would produce, which would show matching changes nearly simultaneously even with detectors thousands of miles apart. The differing placement also helps determine the direction of the waves' source, which travel the speed of light, and their differing attitudes (the directions of the light paths) contributes to this as well as it gives them differing sensitivity to the wave amplitude (gravitational wave strain) based on the direction of the waves' source.
Current examples are the two LIGO detectors, and Virgo, which have been successful detecting recognizable gravitational waves and KAGRA, which became operational in February 2020. These are all on or under ground. Similar space-based detectors using much more length have been proposed but not deployed. They aim at a lower frequency. Two other kinds of detection are being attempted through analysis of more conventional electromagnetic radiation: observation are pulsar timing arrays, and CMB studies.