Aeon : A unit of time equal to one billion years (1 Ga). Proposed in 1957 for use in geology, the aeon is not approved by the SI and hasn't found much favor
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}