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, or twice the age of the universe.
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 attosecond is equal to 1000 zeptoseconds, or 1/1000 of a femtosecond. Because the next higher SI unit for time is the femtosecond (10−15 seconds), durations of 10−17 s and 10−16 s will typically be expressed as tens or hundreds of attoseconds: