A measure of image resolution - typical screen resolution is 72 dip whereas printing presse print at 300dpi.

When measuring the resolution of an image you need to know both the physical size and the resolution (in DPI) to acertain the actual amount of data. For example:

An image that is 10 inches wide...

at 72dpi would be 720 pixels wide
at 300dpi would be 3000 pixels wide (roughly 3.5 times the resolution)