What exactly is a retina display?
Retina displays
To user Philip Schiller's own words from the WWDC2012:

...which means: The pixels of this display are so small that from a
normal working distance your retina cannot discern those individual
pixels. (watch at youtube)

There is no industry standard for &quot;retina display&quot;. The term retina display is relative, as the definition follows the human eyes's perception of a display's sharpness.
When the iPhone 4 was introduced, Steve Jobs made the following definition:

At a distance of 10&quot; to the human eye (or retina), a pixel density of 300PPI is the maximum that an eye can distinguish.

Because this definition is relative, calling a display a retina display depends on two factors:

pixel density
distance between the human eye an the display

Both the iPhone 4S and the new iPad have &quot;retina displays&quot; when you apply Steve Job's definition. A mathematical explanation can be found in this answer.