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")
Kes[]
Security Relay Station One was a manned station operated by the Kes of Kesprytt III in 2370. (TNG: "Attached")
Cardassian[]
In 2373, during the Dominion War, General Martok destroyed a relay station at Felton Prime, managing to do so before the Cardassians even realized he was there. (DS9: "Once More Unto the Breach")
Hirogen[]
A Hirogen relay station
In 2374, the USS Voyager explored a station that was part of a an extensive network controlled by the Hirogen. The station used a quantum singularity as a power source and could be accessed by subspace transceivers. (VOY: "Message in a Bottle", "Hunters")
Federation[]
A 24th century Federation relay station
A 31st century Federation relay station
In 3189, a Federation relay station was administered by an Aditya Sahil, in his capacity as a Federation Liaison officer. As a result of the legacy of the galaxy wide "Burn", its range was reduced to 30 sectors and its ability to communicate with other stations, Starfleet Command or the government of the Federation was significantly curtailed (DIS: "That Hope Is You, Part 1")