Subdermal implant
From Memory Alpha, the free Star Trek reference.
A subdermal implant was a device that was implanted under the skin. They can have various purposes.
Kuroda Lor-ehn had subdermal implants installed on his wrists, designed to depolarize triburnium alloys. This enabled him to escape his restraints on an Enolian transport in 2152. (ENT: "Canamar")
During the Occupation of Bajor, members of the Bajoran Resistance had subdermal implants composed of tritonium isotopes. In the event of capture, the implants could be activated to leave a trail that could be followed. In 2372, Major Kira Nerys used such a trail to find the survivors of the Ravinok. (DS9: "Indiscretion")
