A subcutaneous transponder (also known as subdermal transceiver or subcutaneous communicator) is a small transponder inserted under the skin, which is used by Starfleet and others. Its function is to maintain contact between an away team member and their home ship, and some models allow the user to receive discreet audio transmissions audible only to them, making them particularly useful for undercover work.
In the 22nd century, Earth MACOs utilized subdermal transceivers for covert communications. Captain Jonathan Archer used one to communicate with T'Pol and Hoshi Sato in the command center on the Enterprise NX-01 when he was tricking Degra into giving him the location of the Xindi superweapon. (ENT: "Stratagem")
On a mission to Ekos in 2268, Captain Kirk and Spock had subcutaneous transponders implanted in their arms as a precaution in case they couldn't use their communicators. When they were imprisoned while on that mission, Spock fashioned a crude laser beam using an electric light, metal bands from a cot, and the rubindium crystals in his transponder. The beam was able to cut through the prison cell door lock. (TOS: "Patterns of Force")
Subcutaneous communicators were used by William T. Riker and Deanna Troi in 2366 during a mission to investigate potential cultural contamination to the primitive culture on Mintaka III. (TNG: "Who Watches The Watchers")
In 2377, Chakotay and Neelix were implanted with subdermal transponders for their undercover mission to Quarra, where much of the USS Voyager crew had been brainwashed into the local workforce. The transponders allowed Voyager to beam them out through Quarra's shield grid. (VOY: "Workforce")