A unit of relative amount of substance equal to 10-6 equivalent weight. This unit is used, for example, in stating the concentrations of ions in drinking water
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}