Retina Display, or later Retina HD Display on release of the iPhone 6 and the 6 Plus in September 2014, and Retina 5K Display on release of the October 2014 iMac are brand names used by Apple for screens that have a higher pixel density than their previous models. The terms are used for several Apple products, including the iPhone, iPod Touch, iPad, MacBook Pro, iPad Mini, iPad Air and iMac. Because the typical viewing distance is different, depending on each device's use, the pixels per inch claimed to be of Retina quality can differ, depending on the size of the display, with higher PPI for smaller displays and lower PPI for larger displays. Later device versions have an even higher quality improvement, either counted by an increase in the PPI and/or by an increase in the number of pixels, thus Apple using the moniker "Retina HD Display" or "Retina 5K Display": 401 PPI: iPhone 6 Plus 326 PPI: iPhone 4, iPhone 4S, iPhone 5, iPhone 5C, iPhone 5S, iPhone 6 iPod Touch