6th July 2010
A retina display is one which a human cannot distinguish between the individual pixels. There is already there an argument around whether the iPhone 4's screen is a true 'retina display'. But I think non-scientists can agree that it is pretty darn close.
The crisp display is achieved by by adding extra pixels. For every pixel on the original iPhone, the iPhone 4 has 4. Twice as many wide and twice as many high. This means that the iPhone Operating System, photographs, and web pages appear far crisper than on previous iPhone incarnations.
Strangely, one side effect of this is that as web pages still take up the same amount of space on the screen, a 960 pixel wide web site will actually be 1920 pixels wide on the retina display.
In effect, the pixel just became a relative term. Future mobile devices may further increase the rate of 'pixels per pixel' even further with higher quality displays as they try to trump the Apple device.