The iPhone 4 was the first Apple product to feature a Retina display.
Apple introduced the display with the iPhone 4 in June 2010, pairing a 3.5-inch screen with a 960-by-640 resolution and a density of 326 pixels per inch. That was four times the pixel count of the earlier iPhone 3GS at the same physical screen size, making text and images appear much sharper.
The Retina name referred to Apple's claim that, at a typical viewing distance, individual pixels were difficult for the human eye to distinguish. The marketing idea was influential, although display experts noted that visual acuity varies and that the claim was not an absolute scientific limit.
The fourth-generation iPod touch also received a Retina display later in 2010, followed by the third-generation iPad in 2012. That timing creates a common confusion: the iPad 3 was an early Retina iPad, but not the first Apple product with the technology.