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atto-

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  1. a combining form that enters into compounds denoting a unit one-quintillionth (10−18) the size of the unit named by the stem.
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Origin of atto-

< Danish or Norwegian att(en) eighteen + -o-
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for atto-

atto-

prefix
  1. denoting 10 –18attotesla Symbol: a
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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

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).

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

atto-

pref.
  1. One quintillionth (10-18):attometer.
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The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.