A cranial implant was a device that was installed directly into an individual's brain.
Garak created a control for his implant and used it to relieve the pain of being exiled to Deep Space 9. After continuous use over a two-year period, his body became dependent on the higher level of endorphins, so that when the implant malfunctioned in late 2370, it nearly killed him. In particular, the malfuctioning of the implant damaged his leukocytes (a type of white blood cell) and caused a build up of toxins in his lymphatic system. Doctor Julian Bashir was able to synthesize Cardassian-compatible leukocytes to replace Garak's damaged ones after receiving information on Cardassian biochemistry with assistance from Enabran Tain, who had been the head of the Obsidian Order at the time Garak received his implant. (DS9: "The Wire")
Borg drones possessed a variety of cranial implants. In 2374, Captain Kathryn Janeway accessed Seven of Nine's cranial implant with a microfilament to activate her Borg bioelectric field, enabling her to escape a cell on the USS Dauntless. (VOY: "Hope and Fear")
One of the Borg drone's cranial implants was an interplexing beacon. In 2375, Seven of Nine received a set of phase corrections through this cranial implant during USS Voyager's quantum slipstream flight, which collapsed the slipstream and saved Voyager from destruction. The message had been sent by a future Harry Kim, from an alternate timeline. (VOY: "Timeless")