atto

Atto is a decimal prefix in the International System of Units (SI) representing a factor of 10^-18. Its symbol is 'a'. When attached to a unit, it denotes one quintillionth of that unit (1e-18). It sits between femto (10^-15) and zepto (10^-21) in the SI prefix series. Attosecond (10^-18 s) and attometer (10^-18 m) are among the common derived units using this prefix. Other examples include attogram (10^-18 g), attoliter (10^-18 L), and attowatt (10^-18 W). These units are used in fields that require measurements at extremely small scales, such as ultrafast physics, quantum mechanics, and nanoscale science. In practice, the prefix is applied across units without changing the unit's dimension. The prefix was introduced to the SI in the 1960s as part of the system of prefixes. The name 'atto' is standardized by SI conventions. See also other prefixes such as femto- and zepto-.