The attosecond (symbol: as) is a unit of time. It is equal to one quintillionth of one second (or 0.000 000 000 000 000 001 seconds). It can also be written as 10−18 seconds. Because it can be hard to think about how short this period of time is, on this page is a list of descriptions of periods of time longer than 1 attosecond, but shorter than one femtosecond (which is 10−15 seconds). This should help when thinking about periods of time that have different orders of magnitude.