German symbol for a turn or revolution (Umdrehung), usually seen in combinations such as U/min, the German equivalent of the English symbol rpm for revolutions per minute
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}