A Retina display is a branded series of IPS LCD and OLED displays by Apple Inc. that have a higher pixel density than its traditional displays. The term "Retina" is a trademark registered by Apple, and there is no concrete definition for it. In general, a Retina display is any screen that Apple has decided to call that, and it has a pixel density high enough that Apple has determined that the human eye wont be able to make out individual pixels at a certain viewing distance. The goal is to market a smoother and more natural-looking screen than the non-Retina alternatives on other devices.

Apple has registered the term "Retina" as a trademark and has used various marketing terms to differentiate between its LCD and OLED displays having various resolutions, contrast levels, color reproduction, or refresh rates. Apple has brought out a number of variations on the Retina name over the years, including Retina HD, Liquid Retina HD, Liquid Retina XDR, Retina 4K, Retina 4.5K, Retina 5K, Retina 6K, Super Retina HD, and Super Retina XDR.

The effect of a Retina display is to make everything look crisper, and text in particular benefits from Retina – it looks smoother, with the curves on characters looking like curves instead of jagged steps. Apple "converts" a devices display to Retina by doubling the number of pixels vertically and horizontally, meaning it has four times as many pixels as its non-Retina counterpart.

In summary, a Retina display is a high-resolution screen technology used by Apple on various models of the iPhone, iPad, and other Apple products. It has a higher pixel density than traditional displays, making everything look crisper and smoother. The term "Retina" is a trademark registered by Apple, and there is no concrete definition for it.