An automated or crewed station that receives and retransmits subspace signals, forming the network that lets Starfleet ships communicate with headquarters across interstellar distances.
Subspace messages weaken with distance, so the Federation maintains chains of relay stations and buoys to pass them on. Where the network is thin, a ship may wait days or weeks for a reply from Starfleet Command, a fact that leaves many captains to act alone.
Enterprise shows the start of the system. In Silent Enemy, the NX-01 drops subspace amplifiers named Echo One and Echo Two to keep in touch with Earth. In The Next Generation episode Aquiel, the Enterprise-D investigates an apparent murder on Relay Station 47, a lonely two-person post near the Klingon border.
In the Deep Space Nine episode Destiny, Cardassian scientists help install a relay to allow communication through the wormhole. Voyager's crew borrow an alien relay network to reach home in Message in a Bottle.