An attosecond (as) is a very short period of time. It is equal to one quintillionth of a second (or 0.000000000000000001 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.
1 attosecond: It takes 1 attosecond for light to travel the length of three hydrogen atoms.
1 attosecond: It takes 1 attosecond for an atomic nucleus to react when another particle hits it.
150 attoseconds: It takes 150 attoseconds for an electron to circle the nucleus of an atom.
320 attoseconds: It takes about 300 attoseconds for an electron to jump from one atom to another.