High DPI (dots per inch) screen resolutions, e.g. 4k/UHD/QHD screens in a relatively small form factor, e.g. laptops/tablets. Use this tag for issues relating to the Ubuntu experience on HDPI screens.
Largely because of Apple, the term "HiDPI" is also becoming popular (maybe more popular - at any rate, it is easier to read than "HDPI" for me).