An attosecond is 1×10 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 derived from the Danish word for eighteen (atten). Its symbol is as.

An attosecond is equal to 1000 zeptoseconds, or of a femtosecond. Because the next higher SI unit for time is the femtosecond (10 seconds), durations of 10 s and 10 s will typically be expressed as tens or hundreds of attoseconds:

Times which can be expressed in attoseconds:
  • 0.35 attoseconds: the time it takes for light to travel the diameter of a hydrogen atom.
  • 24 attoseconds: the atomic unit of time
  • 43 attoseconds: the shortest pulses of laser light yet created
  • 53 attoseconds: the second-shortest pulses of laser light created
  • 82 attoseconds (approximately): half-life of beryllium-8, maximum time available for the triple-alpha process for the synthesis of carbon and heavier elements in stars
  • 84 attoseconds: the approximate half-life of a neutral pion
  • 100 attoseconds: fastest-ever view of molecular motion
  • 320 attoseconds: estimated time it takes electrons to transfer between atoms