A subatomic particle with a neutral charge that, together with protons, makes up the nuclei of most atoms. The only atomic nucleus without neutrons is the hydrogen nucleus, which is made up of only a proton. Neutrons and protons, also called nucleons, are held together in the nucleus by a strong nuclear force. The presence of neutrons is essential for the stability of the nucleus, but their number can vary even within the same chemical element; atoms with the same atomic number but different mass numbers, and therefore different numbers of neutrons in the nucleus, are called isotopes. Isotopes can have different physical properties, and some can be unstable.

A free neutron is subject to beta decay with a half-life of about ten minutes. A stream of high-energy neutrons is a type of ionizing radiation. After striking other atomic nuclei, the neutron can be absorbed, forming an unstable isotope. This neutron capture can activate materials that are exposed to strong neutron fluxes, such as reactor chambers in a nuclear power plant or a thermonuclear fusion research facility. A neutron can also cause the nucleus of a heavy atom to fission. This principle is essential for the operation of nuclear power plants, where neutrons fission the nuclei of uranium, plutonium, or thorium. This releases large amounts of energy. Depending on their energy, neutrons can be divided into slow, thermal neutrons, which are in thermodynamic equilibrium with their surroundings, and fast neutrons, with energies above about 1 keV.