Fissile material refers to substances capable of sustaining a nuclear fission chain reaction. In a nuclear reactor or a nuclear weapon, fissile materials undergo nuclear fission, splitting into smaller nuclei when bombarded by neutrons, releasing energy and additional neutrons. These additional neutrons can then trigger further fission reactions, creating a self-sustaining chain reaction.

The most common fissile material used in nuclear reactors is uranium-235 (^235U). Uranium-235 is naturally occurring but relatively rare, constituting only about 0.7% of naturally-occurring uranium. It's enriched to higher concentrations for use in nuclear reactors or weapons. Another fissile material is plutonium-239 (^239Pu), which is artificially produced by irradiating uranium-238 (^238U) in a nuclear reactor. Plutonium-239 is also used in both nuclear reactors and weapons.

Fissile materials play a critical role in nuclear energy production, as they provide the fuel for nuclear reactors. They are also the primary components of nuclear weapons. Due to their potential for releasing vast amounts of energy, fissile materials are carefully regulated to prevent their misuse and proliferation for weapons purposes.