Fissile material

In nuclear engineering, a fissile material is one that is capable of sustaining a chain reaction of nuclear fission. In the arms control context, the term "fissile" is used to describe materials that can be used in the fission primary of a nuclear weapon. These are materials that sustain an explosive fast fission chain reaction.
Uranium-233, Uranium-235, Plutonium-239 and Plutonium-241 are fissile materials.