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a combining form that enters into compounds denoting a unit one-quintillionth (10 −18) the size of the unit named by the stem.
Origin of atto-
< Danish or Norwegian att(en) eighteen + -o- Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2018.
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British Dictionary definitions for atto-


denoting 10–18: attotesla, a
Word Origin
from Norwegian, Danish atten eighteen
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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Word Origin and History for 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).

Online Etymology Dictionary, © 2010 Douglas Harper
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atto- in Medicine

atto- pref.
One quintillionth (10-18): attometer.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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