Retina display
Retina display refers to a high resolution digital display. It has standardize on 264 pixels per inch (ppi) for its iPads
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[edit] Overview
Apple quote: Text in books, web pages, and email is crisp at any size. Images in movies and photos are stunning at almost any angle. That’s because the Retina display’s pixel density is so high, your eye is unable to distinguish individual pixels.
[edit] Calculating
The calculation is related to pixel density, ppi and distance from the eye to the screen. See Angular size calculator to calculate this number.
To understand if the answer is truly a retina display you need to understand visual acuity. However there is no question that Apple retina display are high resolution.
See also The reference pixel on W3C site
[edit] Legality
The term "Retina" when applied to computer display is a registered trademark of Apple.
[edit] Is it needed?
Users will differ on whether it is useful, important, or perhaps absolutely necessary. Some claim that they get headaches on any other type of display while others say they can't tell much difference in actual use.
[edit] Devices
Apple has standardize its iPads at 264 ppi. However, the iPad mini has a 326 ppi resolution which matches the pixel size of a 9.7" Apple iPad. Other smaller devices have standardized on the 326 ppi for all iPhones except the S models which increased the ppi to 401 primarily to reach the standard FHD resolution. The higher resolution is expected for devices that are expected to be used closer to your face.
See the iPad 3 and later, iPad Air and later, iPad mini, iPhone, iPhone 5, and iPhone 6 for devices offering a Retina display.
