Atto-

Atto (symbol a) is a unit prefix in the metric system denoting a factor of 10−18 or 0.000000000000000001. Atto (symbol a) is a unit prefix in the metric system denoting a factor of 10−18 or 0.000000000000000001. 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'.