4K is a display standard often included in 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 (i.e., 3,840 pixels wide (wide) by 2,160 pixels tall (high)). It is exactly double the resolution of HDTV (1920 x 1080) and includes an identical 16 x 9 aspect ratio.
The first mass production of 4K televisions began in 2013. Later, many companies such as Sony, Panasonic, Samsung, LG, Sharp, Xiaomi, and other manufacturers also offer 4K televisions with their HDTV lineups.
Many companies had released high-resolution video capture devices before 2013 to make 4K video content available for new TVs.
For example, Canon, JVC, and other companies released 4K digital video cameras in 2012. At the same time, RED also released RED ONE only in 2007, which later made new avenues for development in the next few years for 4K devices.
While you may have heard 4K often for television using a reference, it also refers to high-resolution computer monitors.
For example, many hardware manufacturers now offer 4K displays called Hi-DPI monitors and retina displays. The most popular Hi-DPI monitors resolution in this is 3840 x 2160, while in some displays, you can get a wider aspect ratio and resolution of about 4096 x 2160.
NOTE 4K televisions should be called UHDTVs (not HDTVs). However, the phrase “4K television” is more commonly used in marketing.