Uranium enrichment refers to production of a quantity of uranium that is "rich" in the specific isotope U-235, which is about 0.7% of natural uranium. This isotope of uranium is fissile, splitting into two smaller atoms and some neutrons when struck by a neutron. The production of additional neutrons means that the material, when in a suitable geometry that promotes the chance of subsequent neutron impact, can produce a self-sustaining chain reaction of successive fissions. This makes it suitable for use in power reactors, and at very high fractions of U-235, in weapons such as the Hiroshima bomb.

A byproduct of producing enriched uranium is that depleted uranium - low in U-235 - is also produced.