The official unemployment rate is defined as "total unemployed, as a percent of the civilian labor force," but doesn't include a number of situations in which workers may find themselves. The U-6 rate is defined as all unemployed, plus "persons marginally attached to the labor force, plus total employed part time for economic reasons, as a percent of the labor force."
The U-6 rate has shown improvement in the past few years, and it fell to 9.6 percent in June, the lowest level since April 2008. It's been more volatile that the U-3 rate. The U-6 is down 70 basis points over the past year, versus a 40-point drop in the U-3.
16:32 Employers added solid 178,000 jobs in Nov8
14:36 U.S. economy added 178,000 jobs in November; unemployment rate dropped to 4.6 percent13
14:49 Trump warns of ‘retribution’ for companies that offshore jobs, threatening 35 percent tariff6
13:41 Solid Job Growth Strengthens Prospect for a Fed Rate Increase10