Retina Display is a term, or brand, coined by Apple for display screens on the iPad, iPhone, iPod Touch, and MacBook Pro. It means that a display has a high enough number of pixels per inch, or PPI, that the human eye is unable to detect any pixelation at standard viewing distances.
The PPI qualifying a screen as a Retina Display varies from device to device as each one has a different viewing distance and screen size. For smaller devices, like the iPhone, there are 326 pixels per inch. The iPad, having a larger screen than the iPhone, has 264 pixels per inch, and the MacBook Pro, having the largest screen, has 220 pixels per inch.
Most human eyes cannot observe detains above 300 PPI at a distance of 12-inches.