A small tracking device injected beneath the skin so that a starship can locate and beam up a landing party member who has lost his communicator.
Subcutaneous transponders are used in the episode Patterns of Force. Before Kirk and Spock beam down to Ekos to look for the missing historian John Gill, McCoy injects one into each man's forearm so that the Enterprise can find them if they cannot call.
The precaution proves useful in a way nobody planned. Captured and whipped by the planet's Nazi-style security forces, the two are locked in a cell without any equipment.
Spock realises that the rubindium crystals inside the transponders can be made to produce a crude laser. They dig the devices out of their arms, and with Spock standing on Kirk's flogged back to reach the cell's light bulb, they focus its light through the crystals and burn out the lock.