A subcutaneous transponder (or subdermal transponder) 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.
When Captain Kirk and Mr. Spock were imprisoned on Ekos in 2268, Spock fashioned a crude laser beam using an electric light, metal bands from a cot, and the rubindium crystals in his subcutaneous transponder. The beam was able to cut through the prison cell door lock. (TOS: "Patterns of Force")
Subcutaneous transponders 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")