In SI, the decimal submultiplier prefix meaning 10-18, or one quintillionth. Symbol, a. The growth of nuclear physics created a need for such a prefix and the physicists were the first to recognize it officially, when the General Assembly of the International Union for Pure and Applied Physics approved a proposal of its Commission on Symbols, Units and Nomenclature in 1960. Use of the atto- with SI units was officially authorized by resolution 8 of the 12th CGPM in 1964.
The prefix comes from the Danish word for eighteen.
Comptes Rendus des Séances de la Conférence Generale des Poids et Mesures, 94.
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