In Star Trek, subspace radio carries messages faster than light through subspace.
Subspace radio is a communications technology that transmits signals through subspace rather than ordinary electromagnetic space. Because subspace communication is not limited in the same way as conventional radio, starships and planets can exchange messages across interstellar distances without waiting years for a light-speed signal.
The system is not the same as a subspace relay or communications array. A radio is the transmitting and receiving method, while relays and arrays are infrastructure that can extend coverage or improve signal strength. Star Trek also distinguishes subspace communication from transporter transmission and warp travel.
Messages can still be delayed, interrupted, or distorted. Distance, interference, local conditions, and the availability of relay stations affect communications. This allows stories to include realistic communication problems while preserving the franchise's faster-than-light setting.