Dots Per Inch (referring to on a line, and not a square measure), a common unit of quality in a picture.
Some common DPI measurements:
- Windows monitors (Default): 96 DPI
- Mac monitors (Default): 72 DPI
As a note (source):
The DPI measurement of a printer often needs to be considerably higher than the pixels per inch (PPI) measurement of a video display in order to produce similar-quality output. This is due to the limited range of colours for each dot typically available on a printer. At each dot position, the simplest type of colour printer can print no dot, or a dot consisting of a fixed volume of ink in each of four colour channels (typically CMYK with cyan, magenta, yellow and black ink) or 24 = 16 colours on laser, wax and most inkjet printers.