Technically, anything that breaks the PAL barrier of 625 lines can be called high definition. The most common HD formats feature 720 and 1080 scan lines.
Ultra High Definition
Ultra high definition features an amazing 4,520 lines of horizontal resolution. Known as “4k” technology because the scan lines exceed 4,000, it will no doubt be the future industry standard.
The following photos show the relative size of the different formats. The first one represents the typical digital video frame (DV and DVCAM). Notice how detail improves as the number of scan lines increases. The final photo illustrates the huge leap in image detail 4k technology provides.
As a point of reference, the typical flat computer monitor has 2,000 lines of resolution. 35mm film–as perceived by the human eye–falls in the mid HD range. For more on 35mm