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.
Note: Most human eyes cannot observe detains above 300 PPI at a distance of 12-inches.