A dead pixel is a picture element in which all three RGB (red, green, blue) sub-pixels are permanently turned off, which creates a black spot in the display.
Malfunctioning transistors and manufacturing flaws can result in dead pixels. Dead pixels are often confused with stuck pixels, which have a different cause and effect. Each pixel in a display is made up of three sub-pixels: one red, one green and one blue. In a stuck pixel, one or more of the three remains turned on so that the pixel displays consistently as a particular color rather than refreshing along with other display content.
Depending on their number and position on the screen, dead and/or stuck pixels can impact the user experience. Both dead pixels and stuck pixels have been reported to disappear (and sometimes reappear) over time. Although stuck pixels are thought to be easier to address than dead ones, a number of DIY fixes and software products offer help for both issues.
NCIX Tech Tips explains dead pixels, how to find them and what to do about them: