Retina Display.

Retina Display is a brand name used by Apple for liquid crystal displays which they claim have a high enough pixel density that the human eye is unable to notice pixelation at a typical viewing distance. The term is used for several Apple products, including the iPhone, iPod Touch, iPad, and MacBook Pro. As the typical viewing distance would be different depending on each device's usage, the pixels per inch claimed as retina quality can be different for the smallest devices (326, iPhone and iPod Touch): greater than the mid-sized devices (264, iPad) and greater than the larger devices (220, MacBook Pro). When an Apple product has retina display, each user interface widget is doubled in width and height to compensate for the smaller pixels. This mode is referred to as HiDPI mode by Apple.
Apple has applied to register the term "Retina" as a trademark in regard to computers and mobile devices with the United States Patent and Trademark Office, Canadian Intellectual Property Office, and in Jamaica. On November 27, 2012 the US Patent and Trademark office approved Apple's application and Retina® is now a registered trademark.

Technical definition

When introducing the iPhone 4, Steve Jobs said the magic number for a retina display is about 300 PPI for a device held 10 to 12 inches from the eye. One way of expressing this as a unit is Pixels Per Degree (PPD) which takes into account both the screen resolution and the distance from which the device is viewed. Based on Jobs' magic number of 300, the threshold for a Retina Display starts at a PPD value of 53. 53 PPD means that a tall skinny triangle with a height equal to the viewing distance and a top angle of one degree will have a base on the device's screen that covers 53 pixels. Any display's viewing quality (from phone displays to huge projectors) can be described with this size-independent universal parameter. Note that the PPD parameter is not an intrinsic parameter of the display itself, unlike absolute pixel resolution (e.g. 1024 x 800 pixels) or relative pixel density (e.g. 72 PPI), but is dependent on the distance between the display and the eye of the person (or lens of the device) viewing the display; moving the eye closer to the display reduces the PPD, and moving away from it increases the PPD in proportion to the distance. It can be calculated by multiplying the distance to the screen times the PPI resolution of the screen times π divided by 180 degrees