Ultra HD TV, also referred to as 4k, UHD, Ultra HD, and sometimes Quad HD, is a video format with a resolution of 3840 x 2160 (8.3 megapixels) that is four-times the Full HD standard of 1920 x 1080p (2.1 megapixels). Current HDTV standards also fall into two lower video resolutions that include 720p and 1080i, but those formats contain either fewer lines of resolution (720p) or are interlaced rather than progressive (1080i).
4k resolution was defined on Oct. 17, 2012 when the CEA (Consumer Electronics Association) announced the term Ultra HD (Ultra High Definition) would be used for displays with a video resolution of 3840 x 2160. But in most cases, consumer electronics manufacturers refer to Ultra HD sets also as 4k TVs, as the term is used more widely to describe the technology.
The next jump in video resolution is 8k UHDTV at 7680 x 4320 (33.2 megapixels) which is 16-times the current HDTV standard.
Here is a chart that illustrates the different format sizes.