An ending used to indicate the number of folds or layers in an object; thus 4-ply means having 4 layers. The suffix comes from the Latin plicare, to fold
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}