The VISOR, or Visual Instrument and Sensory Organ Replacement, was the device that let Geordi La Forge, blind from birth, perceive the world by reading much of the electromagnetic spectrum.
Worn over the eyes and connected to neural implants at the temples, the VISOR fed signals directly to La Forge's brain. It allowed him to see far beyond normal human vision, detecting infrared, ultraviolet and many other emissions, which often made him the first to notice something wrong in an engine or a person.
The device came at a cost. It caused La Forge constant pain, and he declined several offers of surgery that might have restored more ordinary sight because he did not want to lose what the VISOR let him see.
The link to his brain also proved a vulnerability. In 2367 the Romulans captured La Forge and used his VISOR to brainwash him into an assassin, a plot uncovered by Data. By 2373 he had replaced the VISOR with ocular implants.