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Douglas Harper's Etymology Dictionary

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).


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


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.