A metric prefix standing for 10-15 (one quadrillionth). The prefix was coined from femten, the word for fifteen in Danish and Norwegian. This was an attractive idea because it preserved the established symbol for 10-15 meter (see next entry)
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}