Depleted uranium is uranium that contains a lower amount of the radioactive isotope U-235 than natural uranium. The latter usually contains an amount of 0.7 percent U-235, while in depleted uranium, this goes down to 0.3 percent or less.
Depleted uranium is formed during the process of uranium enrichment, which consists of increasing the relative amount of U-235 in a portion of the material. Depleted uranium is formed as a byproduct of this process and represents the portion of the material from which the U-235 is extracted and concentrated in the enriched uranium. Depleted uranium has found many military uses due to its high durability and density.