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a combining form used in the names of units of measurement which are 10− 15 (one quadrillionth) smaller than the unit denoted by the base word.
Origin of femto-
< Danish, Norwegian femt(en) fifteen + -o- Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2017.
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British Dictionary definitions for femto-


denoting 10–15: femtometer, f
Word Origin
from Danish or Norwegian femten fifteen
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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femto- in Medicine

femto- pref.
One quadrillionth (10-15): femtovolt.

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