Dictionary.com
definitions

atto-

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

Nearby words

  1. attius,
  2. attleboro,
  3. attlee,
  4. attn,
  5. attn.,
  6. attolaser,
  7. attophysics,
  8. attorn,
  9. attorney,
  10. attorney general

Origin of atto-

< Danish or Norwegian att(en) eighteen + -o-
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

British Dictionary definitions for atto-

atto-

prefix

denoting 10 –18attotesla Symbol: a

Word Origin for atto-

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-

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

Medicine definitions for atto-

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.