A dead pixel is a defective pixel that remains unlit on an LCD screen, monitor, Camera Charge-coupled device or CMOS sensor.
The term "dead pixel" is often erroneously applied to other defective pixels, but they have separate terms. A permanently lit (white) pixel is called a hot pixel, and a pixel that stays on a solid color (red, green, or blue) is known as a stuck pixel.
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-colored 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 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.
In some cases, the manufacturer sends all screens to sale, and then replaces the screen if the customer reports the unit as faulty and the dead pixels meet their minimum requirements for return.
Recently, this has become a hot button topic with the Sony Playstation Portable system as many units have shipped to sale with dead pixels to varying degrees, and there has not been a clear policy on return requirements (see that article's Criticisms section).
The majority of dead pixels are only noticeable on a solid color background, for example through the use of the "Dead Pixel Checkers" which consist of 4-5 solid-color images.