What does the term 'Retina display' refer to?

The term 'Retina display' refers to a display technology characterized by a high pixel density, which results in text and images appearing sharper to the human eye. When Apple introduced this technology, the key emphasis was on the clarity and resolution that surpasses the typical limits of human visual perception. This means that pixels are packed so closely together that they become virtually indistinguishable at normal viewing distances. As a result, users experience crisper text, more vibrant colors, and more detailed images, enhancing the overall visual experience on devices such as iPhones, iPads, and MacBooks.

The other options describe features or technologies but do not accurately capture the essence of what makes a Retina display unique. OLED screens, for instance, describe a different type of display technology known for its contrast and color accuracy, while functionalities like screen expansion and protective features against scratches pertain to different aspects of device usability and hardware protection. Thus, the chosen answer distinctly highlights the primary characteristic of Retina displays that contributes to a superior graphical experience.