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.
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.
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).