We’ve seen the 4K term being used for some time now to describe the new standard in video resolutions. A 4K resolution means that the horizontal resolution has around 4000 pixels. Currently the standard 4K resolution stands at 3840 x 2160, which has four times the amount of pixels of a Full HD resolution.
However, at some point in development, there were also some who have named this new standard as Ultra High Definition (UHD). In fact, the International Telecommunication Union has approved the use of UHD as the standard to define 4K and even 8K resolutions.
To eliminate further confusion on the matter, the Consumer Electronics Association has now also adopted the use of UHD or Ultra HD to replace all naming conventions for 4K. In addition, devices bearing the UHD name must have input capable of transmitting native video at 3840 x 2160 resolution without any upconversion. All screens bearing the UHD standard must also have at least 8 million active pixels, with the minimum resolution being the one mentioned above.
(Source: The Verge)