Display resolution of a computer monitor, tablet, laptop, smartphone or any other display device is the number of distinct pixels in each dimension that can be displayed. High-density monitors or Retina monitors sold by Apple are generally recognized as having a physical pixel density greater than 200 pixels per inch. This means that they have twice the pixel density of a classic resolution computer monitor. 4K monitors for PCs hit the shelves in 2014, and understanding pixel density has become important when choosing products along with screen size and resolution. Pixel density is the specification that defines how detailed a display is. It is expressed in units of PPI (Pixels Per Inch).
The reason that CSS pixel ratio was created is because as phones screens get higher resolutions, if every device still had a CSS pixel ratio of 1 then webpages would render too small to see.
4K monitors for PCs hit the shelves in 2014, and understanding pixel density has become important when choosing products along with screen size and resolution. It is predicted that 4K will replace Full HD as the standard resolution. Of course, 4K stands for 4,000 and refers to a horizontal number of pixels around this number.
DCI 4K has twice the screen resolution of 2048 x 1080 pixels on projectors (4096 x 2160 / approx. 17: 9) and is the 4K industry's 4K resolution. UHD 4K (also called UHDTV 4K), on the other hand, is the 4K resolution of the television industry, which was defined by the International Telecommunication Union (ITU). It has twice the horizontal resolution of 1920 x 1080 pixels Full HD (3840 x 2160/16: 9).