http://blog.typekit.com/2013/05/01/hi-dpi-typography/
What makes a device “hi-DPI” or not. In general, a device is hi-DPI if it has both of these things:
- A display with a (relatively) high pixel density, say, over 200 pixels per inch (PPI) for mobile devices or over 150 PPI for laptops.
- Pixel scaling, where the on-screen graphics are rendered using extra physical pixels so that they appear sharper and clearer than on a standard display.