Skull
From Memory Alpha, the free Star Trek reference.
The skull is the framework of the head in most animals including humanoids. The skull protects the brain and has sockets for housing the eyes. It is part of the skeletal system.
Gallamites have transparent skulls. (DS9: "The Maquis, Part I")
Data's skull was composed of cortenide and duranium. (TNG: "The Chase")
Cranial implants are installed into the skull. (DS9: "The Wire")
Both Hirogen and Kazon keep the skulls of slain victims on trophy walls aboard their starships. (VOY: "Initiations", "Hunters", "Prey")
Doctor McCoy also kept several alien skulls in his sickbay. (TOS: "The Conscience of the King")
Quark once had damaged bones in the back of his skull repaired. (DS9: "Take Me Out to the Holosuite")
Terran Empire MACOs used the skull as a symbol. (ENT: "In a Mirror, Darkly")
