A metric prefix denoting 10-18 (one quintillionth). For example, one electron volt equals 0.1602 attojoule. The root of the prefix is atten, the word for 18 in Danish and Norwegian
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}