An eyepiece was a cybernetic device replacing one of the eyes of some Borg drones.
A removable component, the Borg eyepiece was capable of detecting a wide visual spectrum as well as providing advanced spacial and geometric perspectives. Some versions emitted low-intensity lasers. (ENT: "Regeneration"; TNG: "I Borg"; Star Trek: First Contact)
After Seven of Nine was freed from the Borg Collective in 2374, The Doctor designed an artificial eye to replace her eyepiece. (VOY: "The Gift")