In general, an isotope or a mixture of isotopes capable of nuclear fission. Some fissionable materials are capable of fission only by sufficiently fast neutrons (e.g., neutrons of a kinetic energy above 1 MeV). Isotopes that undergo fission by neutrons of all energies, including slow (thermal) neutrons, are usually referred to as fissile materials or fissile isotopes. For example, isotopes ²³³U, ²³⁵U, ²³⁹Pu and ²⁴¹Pu are referred to as both fissionable and fissile, while ²³⁸U and ²⁴⁰Pu are fissionable but not fissile.