##### 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.