Definitions

from Wiktionary, Creative Commons Attribution/Share-Alike License

  • n. 10-15 of a metre. Symbol: fm

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

  • n. a metric unit of length equal to one quadrillionth of a meter

Etymologies

femto- +‎ metre (Wiktionary)

Examples

  • The proton and the neutron have a diameter of 10-15m (a femtometre or a millionth of a millionth of a millimetre) and the electron is at least 1,000 times smaller.

    The Nobel Prize in Physics 1992

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