A traditional unit of distance equal to 10-6 inch, 0.001 mil, or 25.4 nanometers (nm). The microinch is used rather widely to state the roughness of optical surfaces, precise tolerances in machining, and for other industrial purposes
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}