pixels per inch (PPI)
Synonyms: pixel density
A measure of the level of detail that can be reproduced by a computer display.
The PPI of a display represents the number of pixels (coloured dots) in a given inch. Normally, the pixels are square and the PPI is the same both horizontally and vertically.
When speaking about PPI, people often substitute the term DPI, which more correctly refers to the level of detail reproduced by a digital printer.
There is a persistent myth that "images on the web are 72DPI". This statement is meaningless, because computer images do not have DPI (or PPI).
PPI is an attribute of the display, not the image. PPI varies from one display to another, from as low as 70 PPI on an older CRT to 300 PPI on a smartphone.
Also see: DPI.