4K

4K is a high-definition video resolution standard allowing about four times the resolution of 1080p signals - up to 4096 x 2160 at 24Hz. Formerly only used for digital projection in theaters, it is becoming a new standard for home theater displays as well.

4K refers to approximately 4,000 pixels of horizontal resolution, in contrast to standard digital TV resolutions which are stated in vertical lines.  If 1080p has 1080 lines of vertical resolution (1920x1080), 4K video has 2160 - double the lines of vertical resolution.

For consumer displays, 4K (also known as Ultra HD [UHD]) video has double the lines of vertical resolution and double the number of pixels of horizontal resolution (3840x2160), maintaining the 16:9 aspect ratio of home-entertainment TVs.

For shooting major motion pictures as well as for cinema theater projection, the film industry likes to shoot at resolutions at least 3-4 times greater than what it will project, so the Digital Cinema Initiative (DCI) backed by the major Hollywood studios came out with a 4K specification (at the same time it came out with the 2K display specification). DCI's 4K has 2160 vertical lines of resolution, which is double that of 1080p, and 4096 pixels of horizontal resolution (4096x2160), which is exactly double that of 2K, thus maintaining the 17:9 aspect ratio that better aligns with several of 35mm and 70mm films formats used to shoot and project movies.