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femto-

  1. 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.
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Origin of femto-

< Danish, Norwegian femt(en) fifteen + -o-
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

British Dictionary definitions for femto-

femto-

prefix
  1. denoting 10 –15femtometer Symbol: f
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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

femto- in Medicine

femto-

pref.
  1. One quadrillionth (10-15):femtovolt.
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The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.