So what actually is HD? HD, or High Definition, refers to the detail level of a screen; or to put it another way, the number of pixels that are packed into a display. A pixel is the smallest visible element on a display, the ‘dots’ that combine to make up the overall picture. High definition commonly refers to an increase in display or visual resolution over a previously used standard.
HD follows from standard definition (the level of detail in analogue colour TV that most of us grew up with), cramming in even more pixels in order to produce sharper, cleaner images when playing video.