An image measurement specifically describing a particular resolution (pixels) at a particular dimension (in this case, inches.)
Example: a 300 ppi image contains 300 pixels per each inch in layout.
PPI is used almost exclusively for print, where the printed dimension is fixed and being reproduced using a halftone dot. By contrast, an image displayed on a digital device may be displayed very large (ie on a TV) or small (ie on a phone.)
There is a strong correlation with Pixels. An image for print could be described as 4″ wide at 300 ppi, while describing the identical image as 1200 pixels wide.
Frequently confused with Lines Per Inch.