In terms of displays, the screen resolution is a measure of the amount of detail which can be shown. Either in terms of the absolute number of pixels in an image, such as 1920×1080 or in terms of the number of pixels contained within a certain area – often expressed as pixels per inch (ppi). Apples Retina displays offer a resolution high enough to exceed the resolution of the human visual system, thereby rending individual pixels invisible and making screen elements appear smooth rather than blocky.