What does 4K or 8K mean in video resolution?

4K and 8K are measurements derived from the cinema standards of measuring video resolution, where the horizontal resolution is approximated to the nearest thousand pixels. A video or sensor with a horizontal resolution of 2048 pixels would be represented as 2K, 2880 pixels is represented as 2.8K, 6560 pixels is represented as 6.5K, etc. In 2005, Digital Cinema Initiatives (DCI) set standardized 2K and 4K containers for cinema distribution. These were set at 2048×1080 and 4096×2160 respectively. Resolutions for consumer video are slightly different than cinema standards, and in 2007 the Society of Motion Picture and Television Engineers (SMPTE) defined standard resolutions for 3840×2160 (also known as 2160p) and 7680×4320 under the names UHDTV1 and UHDTV2.

Several years later when television manufacturers began marketing panels with 2160p/UHDTV1 resolutions, a decision was made to market them as “4K” resolutions instead, adopting the catchier moniker used in cinema standards. Because of this, there are now two resolutions represented by the term “4K”. DCI 4K refers to a resolution of 2048×1080 while UHD 4K represents a resolution of 3840×2160. For anything outside of cinemas and film production, UltraHD 4K (3840×2160) is the common standard when considering the term 4K. The same marketing move has been made for 8K, with the term now commonly referring to the UHDTV2 resolution of 7680×4320.