Display Pixel Density
The number of pixels per inch on a screen, which determines how much image detail it can actually show.
Measured in
ppi
In depth
Density affects how images should be prepared for screen: a high-density display renders a given image at half the physical size unless the operating system scales it, which is why web images are often supplied at twice their nominal dimensions. It also affects how sharpening is judged, since a dense screen shows finer artefacts.
Examples in practice
A web image supplied at 2,048 px displays crisply on a high-density screen where a 1,024 px version looks soft.