A unit of digital information equal to 4 bits or 1/2 byte. This unit, in various contexts, is also called a nibble, a quadbit, or a hexit. The unit seems to be more common in German, where it is spelled tetrade
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}