What does fermion mean?
A fermion is any subatomic particle with half-integer spin that obeys Fermi–Dirac statistics and, crucially, the Pauli exclusion principle — the rule that no two identical fermions may occupy the same quantum state simultaneously. This single constraint underpins much of the structure of matter: it explains why atoms have electron shells, why chemistry exists, and why solid objects cannot pass through one another. Familiar fermions include electrons, quarks, and neutrinos, while composite particles such as protons and neutrons are also fermions because they contain an odd number of quarks. The word honours Italian physicist Enrico Fermi, whose work on statistical mechanics described how such particles behave. In modern physics, every known elementary particle falls into one of two great families — fermions, which make up matter, and bosons, which transmit forces. The term is technical, belonging chiefly to textbooks, research papers, and popular science writing rather than everyday speech.
nounA subatomic particle that obeys Fermi–Dirac statistics and the Pauli exclusion principle, having half-integer spin; fermions include quarks, electrons, and other matter particles.
- Any particle with half-integer spin obeying the Pauli exclusion principle, including electrons, quarks, neutrinos, and composite particles like protons and neutrons.
"Electrons are fermions, which is why no two of them can occupy the same quantum state in an atom."
"In superconductors, two electrons can pair up to behave collectively as a boson rather than as individual fermions."
"The standard model classifies every known elementary particle as either a fermion or a boson."
Regular plural; used when referring to multiple such particles or classes of them.
"All known quarks and leptons are fermions."
You are made entirely of fermions — every atom in your body is held together by the fact that they refuse to share space.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of fermion
The term 'fermion' was coined in honour of the Italian-American physicist Enrico Fermi, whose work in the 1920s on the statistical behaviour of particles led to what is now called Fermi–Dirac statistics. It entered English scientific vocabulary in the mid-twentieth century alongside 'boson', named for Satyendra Nath Bose, its counterpart term for particles with integer spin. The suffix '-on' follows the convention for naming subatomic particles.
Related word forms
How fermion is actually used
A technical term used primarily in physics contexts. Pluralised regularly as 'fermions'.
Easily confused with fermion
Bosons have integer spin and can share quantum states, while fermions have half-integer spin and obey the Pauli exclusion principle.