An attosecond (abbreviated as as) is a unit of time in the International System of Units (SI) equal to 10^-18 or ¹/_{1 000 000 000 000 000 000} (one quintillionth) of a second.

An attosecond is to a second, as a second is to approximately 31.69 billion years.

The attosecond is a tiny unit, but it has various potential applications: it can observe oscillating molecules,