Nuclear fission is a nuclear reaction by which an atomic nucleus splits into two or more smaller nuclei. Fission can be either spontaneous or triggered by a collision with a neutron. Fission is often observed in elements with high atomic numbers, and may release neutrons upon fissioning. Fission always converts a small amount of the nucleus' mass into energy per the equation E=mc2.
An atomic nucleus may, upon collision with a neutron, capture the neutron. The probability of an isotope of capturing a neutron is called its cross section. Generally, the cross section is inversely proportional to the speed of the neutron; the cross section of uranium-233 is many times larger for a low-energy or "thermal" neutron than for a high-energy or "fast" neutron. Fission can happen spontaneously in certain isotopes such as plutonium-240. Fission is triggered more easily when a fast neutron is captured, so many isotopes can actually be fissioned, but not be considered fissile. A nuclear poison is an isotope that has a large cross section but very low probability of fissioning.
An isotope that can fission upon capturing a thermal neutron is called fissile. Generally, an isotope of an element with atomic number above 90 and an odd number of neutrons is fissile. The only naturally-occurring fissile isotope is uranium-235. Fissile material can be created in a process called breeding where fertile material is exposed to neutrons and thus converted into fissile material. Uranium-238 and Thorium-232 are fertile isotopes that can be converted into fissile plutonium-239 and uranium-233, respectively.
A nuclear chain reaction occurs when a fissile isotope fissions, releasing neutrons that cause other fissile isotopes to fission, and so forth. A chain reaction's rate of progression or criticality can either be constant (critical), slowing down (subcritical), or speeding up (supercritical), depending on factors such as the concentration of fissile material (enrichment) or the material's density. Reactors designed for nuclear power are engineered to be critical, while nuclear weapons are supercritical when activated. The amount of fissile material required to initiate and sustain a chain reaction is its critical mass.