Subcutaneous communicator

From Memory Alpha, the free Star Trek reference

A subcutaneous communicator is a communicator implanted below the skin which allows the user to receive discreet audio transmissions, and is particularly useful for undercover work. It differs from a subcutaneous transponder in that it allows actual audio communication.

In 2366 Dr. Crusher implanted subcutaneous communicators in Commander Riker and Counselor Troi when they were posing as Mintakans during their attempt to rescue Captain Picard. (TNG: "Who Watches The Watchers")