Since the introduction of the iPhone 4, Apple has been touting its Retina display as the next big thing in display technology for iPhones, iPads and laptops, but the company doesn't go out of its way to define it. Retina display is Apple's brand name for a display that has a higher resolution than the human eye can discern.
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The term Retina display comes from the concept that the display on the iPad is of a high enough resolution that the human eye cannot discern individual pixels at a normal viewing distance -- defined as roughly 15 inches in the case of the iPad. A study which appeared in the February 1990 issue of the Journal of Comparative Neurology indicated that the average human retina can resolve 0.78 arcminutes per cycle at a 1-foot viewing distance before the optics of the eye are taken into account, or roughly 1 arcminute per cycle including the cornea and lens of the eye. This translates roughly to 287 pixels per inch at a viewing distance of 1 foot.
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The iPad and iPad Air are slightly below this number, at 264 pixels per inch, but are designed to be viewed at a minimum distance of 15 inches, at which these displays are of a higher resolution than the human eye is capable of discerning, meaning no visible pixels. The iPad mini is well over the threshold even at 12 inches with a resolution of 326 pixels per inch.