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
femto- + metre (Wiktionary)
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.