A neural implant was a piece of medical technology surgically implanted into the brain. A neural implant translated delta-compressed wavelength patterns into brainwave patterns that could be understood by the host.
Geordi La Forge had two bilateral neural implants in the temporal regions of his skull that functioned as the bioelectrical interface between his visual cortex and his VISOR. A portion of his implants remained external to allow for the direct connection to his VISOR. By means of these implants, Romulans brainwashing La Forge were able to directly send images and secret commands to his visual cortex. (TNG: "The Mind's Eye")