Loading speed, web and servers
pixel density
In short and in plain language
Pixel density says how finely a screen shows images.
With high pixel density, an image needs more dots to look sharp.
The browser uses it to pick the right size of an image.
Definition
Pixel density states how many physical pixels of a screen correspond to one CSS pixel. On screens with high pixel density, an image needs more pixels in width to look sharp. The browser takes it into account when it picks the right file from several image sizes.