Definition:
Apple calls the display on the iPhone a "Retina Display," saying it offers more pixels than the human eye can see -- a claim that has been disputed by some experts.
The iPhone 4's Retina Display features a resolution of 326dpi (dots per inch). When announcing the phone, Apple's Steve Jobs said that 300dpi is a "magic number," because it's the limit of the human retina to distinguish pixels. Because the iPhone 4's display features a resolution higher than the magic number, Apple says it will always look smooth and clear.