ppcm, ppi [1] : Abbreviations for pixels per centimeter and pixels per inch, respectively. A pixel is a single "picture element", so these units measure the resolution, or fineness, of an image
Russ Rowlett (Director), How Many? A Dictionary of Units of Measurement, Center for Mathematics and Science Education University of North Carolina, USA 2005. http://www.unc.edu/~rowlett/units/dictA.html, (Unit of Meas and Covr Factors - Amer) {608}