A dead pixel is a damaged pixel that permanently remains off because it no longer receives power, possibly due to damaged transistors. A dead pixel can easily be seen on light or white backgrounds since the pixel is always off, hence black. A pixel on a screen is composed of three sub-pixels of red, green and blue, and, sometimes, only one or two of these sub-pixels are dead, making the whole pixel appear as a different color depending on which sub-pixels remain functional.
Techopedia explains Dead Pixel
A dead pixel is a result of a manufacturing defect or imperfection during the making of the LCD screen and may not be immediately evident. A dead pixel can occur later in the life of a screen, when the defective transistor is finally worn out and dies. That is why there is usually no remedy for a truly dead pixel, except to have the whole screen replaced.
A dead pixel is often incorrectly used as a synonym for stuck pixels, which are pixels simply stuck either in the on or in the off state, mostly because they usually have the same appearance. But as a constant, a dead pixel is always off, whereas some stuck pixels can be either off or on, and popular solutions found on the Web can sometimes help in recovering stuck pixels. Another distinction is that all the sub-pixels in a dead pixel are dead as opposed to stuck pixels, which usually have one or two sub-pixels stuck in one state or the other. Dead pixels are becoming increasingly difficult to notice as the resolutions of LCD screens go up and the sizes of individual pixels go down.