Atto- (symbol a) is an SI prefix to a unit and means that it is 10-18 times this unit. Examples are one attosecond or one attometre (U.S. spelling attometer).
Adopted in 1964, it comes from the Danish atten, meaning eighteen.
Last updated: 02-08-2005 13:28:48
Last updated: 05-03-2005 02:30:17