If one is applying for SSDI (Social Security Disability Insurance), then the term "DLI" (date last insured) is important because it is the date by which one must demonstrate that the disabling condition(s) prevented the person from work. Usually, if one is under 50 and has not worked 5 of the last 10 years at the time of applying for disability benefits, the DLI date may become a critical point of inquiry.
Example: Joe is 39 years old and has a DLI of 2/22/2012. He stopped work in 2/2/2010 (severe back pain) but didn't consistently start seeing his doctor util late 2011 regarding his condition. Demonstrating to SSA that his disabling condition prevented him from work before the DLI may be an issue.