Subspace transmitter
A subspace transmitter was any mechanism or method that could transmit information through the medium of subspace and thus at a velocity greater than the speed of light. 

Starships employed such transmitters to stay in contact with their operational bases, landing parties, and away teams. Colonies and civilizations employed them to contact other worlds within the Federation and elsewhere. The Federation maintained a network of subspace relays that were essentially repeaters; these enabled even distant transmitters to reach any point within Federation space.

The Vulcan s of the monastery at P'Jem kept an old transmitter in the catacombs beneath their monastery. According to the Vulcan Elder, this transmitter was very old and hasn't been used in many years. When  was brought to the transmitter and identified its power cell as a krellide power cell, he fixed it and sent  a message. ()

The subspace transmitter aboard the Arcos was failing when it was orbiting Turkana IV in early 2367, meaning that the distress call sent by the ship was very weak and distorted, although the , the ship that received the message, was only eleven minutes away from the Turkana system. ()

de:Subraumtransmitter
Subspace
Communications technology