A dead pixel is a defective pixel that remains unlit, permanently lit, or a solid color on an LCD screen, monitor, Camera Charge-coupled device or CMOS sensor.
In LCD manufacture, it is common for a display to be manufactured that has a number of sub-pixel defects (each pixel is composed of three primary-coloured sub-pixels). The number of faulty pixels tolerated before a screen is rejected is dependent on the "Class" which the manufacturer has given the display (although officially described by the ISO 13406-2 standard, not all manufacturers interpret this the same way, or follow it at all). Some manufacturers such as Samsung have a zero-tolerance policy with regard to LCD screens, rejecting all units found to have any number of sub-pixel or pixel defects, meaning the display is a "Class I" display. Others reject them according to the number of total defects, or the number of defects in a given group, or other definitions. Some screens come with a leaflet stating how many dead pixels they are allowed to have before you can send them back to the manufacturer. Dead pixels can also occur in clusters and these are particularly annoying and in most cases these can be sent back the manufacturer