noun Physics.
any particle that obeys the exclusion principle and Fermi-Dirac statistics; fermions have spins that are half an odd integer: 1/2, 3/2, 5/2, …

1945–50; fermi + (mes)on

fermionic, adjective
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World English Dictionary
fermion (ˈfɜːmɪˌɒn)
Compare boson any of a group of elementary particles, such as a nucleon, that has half-integral spin and obeys Fermi-Dirac statistics
[C20: named after Enrico Fermi; see -on]

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American Heritage
Science Dictionary
fermion   (fûr'mē-ŏn', fěr'-)  Pronunciation Key 
An elementary or composite particle, such as an electron, quark, or proton, whose spin is an integer multiple of 1/2 . Fermions act on each other by exchanging bosons and are subject to the Pauli exclusion principle, which requires that no two fermions be in the same quantum state. Fermions are named after the physicist Enrico Fermi, who along with Paul Dirac developed quantum statistical models of their behavior. Compare boson.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
The dude doesn't even know the difference between a ghost field and a fermion,
  not to mention the other bizarre statements.
At normal temperatures, the difference between boson atoms and fermion atoms
  isn't visible.
Or there can't be the right gauge group with the right fermion spectrum.
By definition, no fermion can be in exactly the same state as another fermion.
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