Decennium : A unit of time equal to 10 years; another name for a decade [2]. The census in the U.S. is called decennial because it is taken every ten years
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}