a prefix utilized in the Metric System of Measurement which denotes 10 to the negative power of 18 (10-18 ) or 0 000 000 000 000 000 001 Its symbol is a
atto
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[ 'a-(")tO ] (combining form.) International Scientific Vocabulary, from Danish or Norwegian atten eighteen, from Old Norse AttjAn; akin to Old English eahtatIene eighteen.