Plotter Unit : A unit of distance used in typography equal to 1/40 millimeter or 25 micrometers. This, the smallest distance addressed by Hewlett Packard plotters, has become a fairly familiar term in digital graphics design
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}