Retina Display
Retina display is a term, or brand, coined by Apple for display screens on the iPad, iPhone, iPod Touch, and MacBook Pro. It means that a display is of high quality enough that the human eye is unable to detect any pixelation at a standard viewing distance. The number of pixels per inch (PPI), in order to be a retina display, can differ per device due to the viewing distance being different for each device. For smaller devices, like the iPhone, there are 326 pixels per inch. The iPad, having a larger screen than the iPhone, has 264 pixels per inch, and the Macbook Pro, having the largest screen, has 220 pixels per inch. Manufacturers that aim for retina quality displays adhere to these guidelines.
Some experts on the human eye have stated that if a person has average eyesight, being 20/20 vision, a picture on a retina display will look good. However, if a person has better than average eyesight, better than 20/20, their eyes will notice the pixelation in the image based on the guidelines that Apple uses for retina display. This is due to the fact that the human retina has a resolution of about 457 pixels per inch, at a distance of 12 inches from the eye. Overall though, most people will not notice the pixelation, making for a stunning image quality on the retina display.
Also see: Apple definitions, Display definitions