See Windows 2000
A digital TV format with 1,080 lines of vertical resolution (1080p). The 2K term came about after 4K TVs debuted in order to contrast 4K with the previous 1080p standard. The 2K refers to the 1920 in the 1920x1080 screen resolution; thus, "2K vs. 4K" and "1080p vs 4K" are synonymous. The "p" is standard today (see progressive scan
Nearly a Century of Tradition Reversed
When 4K TVs came out, the resolution metric was reversed for marketing purposes. Since the inception of television, measurements were given in vertical lines of resolution: 480, 720 and 1080. However, 4K resolution is 3840x2160, and the 3840 horizontal pixels sounds more exciting than 2160 vertical pixels, so 4K was chosen (close enough to 3,840).
2K 1920x1080 (see DTV)
4K 3840x2160 (see 4K TV).
4.5K 4480x2520 (see 4.5K resolution).
5K 5120x2880 (see 5K monitor)
6K 6144x3160 (see 6K resolution)
8K 7680x4320 (see 8K TV)