Find the word definition

Douglas Harper's Etymology Dictionary
atto-

word-forming element meaning "one quintillionth," 1962, from Danish atten "eighteen" (a quintillion is 10 to the 18th power), related to Old English eahtatene (see eighteen).

Wiktionary
atto-

pre. In the International System of Units and other metric systems of units, multiplying the unit to which it is attached by 10−18. Symbol: a

Wikipedia
Atto-

Atto- (symbol a) is a unit prefix in the metric system denoting a factor of 10 or .

The unit multiple was adopted at the 12th General Conference on Weights and Measures (CGPM) in Resolution 8. It is derived from the Danish word , meaning "eighteen".

Examples of its use:

  • Weight of the HIV-1 virus is about or , which can be written as 1 fg or 1000 ag.
  • More examples available.