4K is a display standard that includes televisions, monitors, and other video equipment that supports a horizontal resolution of roughly 4,000 pixels. The most common 4K standard is Ultra HD (or UHD), which has a resolution of 3840 x 2160 pixels (3,840 pixels wide by 2,160 pixels tall). It is exactly twice the resolution of HDTV (1920 x 1080) and has an identical 16 x 9 aspect ratio.
Mass production of 4K televisions began in 2013. Sony, Panasonic, Samsung, LG, Sharp, and other manufacturers now offer 4K televisions alongside their HDTV lineups. Several companies released high-resolution video capture devices prior to 2013 so that 4K video content would be available for the new TVs. For example, Canon, JVC, and other companies released 4K digital video cameras in 2012. RED released the RED ONE in 2007, which paved the way for the 4K devices that followed in the next few years.
While 4K is often used in reference to television, it can also refer to high-resolution computer monitors. For example, several hardware manufacturers now offer 4K displays, which may also be called Hi-DPI monitors or retina displays. The most popular Hi-DPI monitors resolution is 3840 x 2160, though some displays have a wider aspect ratio and a resolution of 4096 x 2160.
NOTE: 4K televisions may also be called UHDTVs (as opposed to HDTVs). However, the phrase "4K television" is more commonly used in marketing.