measure of length that is equal to 10−18, or 1 quintillionth (atto- 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.
There you go shortstuff as clear as mud.
