Above: Close-up of an LCD, showing a dead green subpixel.
Defective pixels are pixels on a liquid crystal display (LCD) not performing as expected. The ISO standard ISO 13406-2 distinguishes between three different types of defective pixel - dark dot defects, bright dot defects and partial sub-pixel defects - while hardware companies tend to have further distinguishing types, such as the stuck sub-pixel.
Similar defects can also occur on CCD or CMOS image sensors in digital cameras. In these devices, defective pixels fail to sense light levels correctly, whereas defective pixels in LCDs fail to reproduce light levels correctly.
Full article at: en.wikipedia.org/wiki/Defective_pixel