A confusing expression of frequency. The word is used both for twice a year and for once every two years, so it should be avoided. Twice a year is semiannually and once every two years is biennially
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}