Pixels per inch

Pixels per inch (PPI, or ppi) is a measurement of the pixel density of the of a bitmap display or video input device.

Pixel densities of Apple products
With the release of the original Macintosh 128K in 1984, Apple strove to achieve a minimum pixel density of 72 pixels per inch (ppi) to allow the implementation of WYSIWYG (what you see is what you get) for printing at 144 dpi. With the introduction of Retina displays, starting with the iPhone 4 in 2010, Apple strove to achieve a minimum density of 220 pixels per inch so that individual pixels were no longer discernable at normal viewing distances.