Uranium having a percentage of uranium-235 smaller than the 0.7 percent found in natural uranium. It is obtained from spent (used) fuel elements or as byproduct tails, or residues, from uranium isotope separation.
Uranium containing less 0.7% uranium-235, the amount found natural uranium (See also enriched uranium.)
http://www.epa.gov/radiation/radionuclides/uranium.htm