1) One quintillionth of a second 2) Time unit 3) Unit of time 
An attosecond is an SI unit of time equal to 10−18 of a second (one quintillionth of a second). For context, an attosecond is to a second what a second is to about 31.71 billion years. The word `attosecond` is formed by the prefix atto and the unit second. Atto- was made from the Danish word for eighteen (atten). Its symbol is as. An attosecon.....
[n] - one quintillionth of a second
attosecond (as) 1. One attosecond (as) is equivalent to the time it takes for light to cross an atom. 2. One quintillionth of a second; one thousandth of a femtosecond. 3. There are 1,000,000,000,000,000,000 (translated as a million trillion) attoseconds in one second. — Source for this atto (as) word: Chemistry and Molecular Biology Resou......
A time of 10-18seconds. See also: Second.
noun one quintillionth (10^-18) of a second; one thousandth of a femtosecond