Also known as UHDTV, UHD, or Ultra High Definition, is a standard announced by the Consumer Electronics Association on October 17, 2012. It encompasses televisions with a 4K (3840 x 2160 pixels) or 8K (7680 x 4320 pixels) screen resolution, and a 16:9 aspect ratio.
These two display resolutions are much more detailed than Full High Definition (also known as 1080p) at four and eight times the pixel count respectively. While 4K televisions are currently mainstream, 8K is still a ways out due to lack of programming, bandwidth consumption, and huge file sizes for stored media. Essentially, there are 8K displays for sale, but they're expensive and the content available is likely half of their native resolution.