"Retina Display" is Apple's marketing term referring to screens whose pixel densities are high enough that the human eye cannot discern the individual pixels. Apple devices with Retina Displays include the iPhone, iPad, MacBook Pro and iMac.
There is no specific figure that defines a Retina Display, because the actual pixel densities in Apple devices vary due to the differences in screen sizes and resolutions. Viewing distance also affects how the human eye can distinguish a single pixel from another. Users typically hold their iPhones closer to their faces than with their iPads, which means iPhone models with Retina Displays have a relatively higher pixel densities than iPads with Retina Displays. Regardless of its actual pixel density, a Retina Display aims to offer better image quality with features such as improved viewing angles, higher contrast and text anti-aliasing.
Apple first used the term "Retina Display" with the launch of the iPhone 4 in June 2010. The iPhone 4 has a 3.5-inch display and a 960-by-640 screen resolution, which means the phone's pixel density is 326 pixels per inch. Variations of the marketing term include the "Retina HD," which describes the displays of the iPhone 6 and iPhone 6 Plus, and "Retina 5K," which is featured in the 2014 27-inch model of the iMac.