A pixel on an LCD monitor that remains unlit, or black, when it should be activated and displaying a color. Each pixel on an LCD screen is made from three separate subpixels-one red, one green and one blue-that when combined form the colors that the users see on the monitor. A dead pixel occurs when the transistor that activates the amount of light that shows through all three subpixels malfunctions and results in a permanently black pixel. Dead pixels are rare and largely go unnoticed by the user.
See "All About Monitors: CRT Vs. LCD" in the Did You Know...? section of Webopedia.
See also "LCD Versus Plasma: An introduction to flat screen television technologies" in Webopedia's Quick Reference section.