A Communication relay station, or simply relay station, was a technical platform designed to receive communications signals and re-transmit them to other stations in a larger communications network. Organizations and species including the Hirogen, the Dominion, the Federation, and the Cardassians used relays as part of networks for communication purposes. Subspace relay stations, using Subspace communication, were common.
These stations could be manned or unmanned, space- or planet-based. Relay stations could be manned by a small crew of as few as two officers. Federation stations were considered as outposts. (TNG: "Aquiel")
In 2374, the USS Voyager discovered several relay stations, parts of an extensive network controled by the Hirogen. The stations used quantum singularity power sources and could be accessed by subspace transceivers. (VOY: "Message in a Bottle", "Hunters")