Atto-

Atto- (symbol a) is a unit prefix in the metric system denoting a factor of 10−18 or 0.000 000  000  000  000  001.

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 atten, meaning "eighteen".

Examples of its use:
 * Weight of the HIV-1 virus is about 1 x 10−18 kg or 1 x 10−15 g, which can be written as 1 fg or 1000 ag.
 * More examples available.