A unit of amount of substance equal to a millionth of a mole (see below). This unit is used very commonly in biochemistry, since a mole of a large organic molecule can be quite a large amount
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}