A new word meaning one billion (109) years. The plural billennia is appearing informally in science writing in phrases such as "countless billennia" or "the billennia of evolutionary time
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}