The natural rate of unemployment (NAIRU) is the rate of unemployment arising from all sources except fluctuations in aggregate demand. Estimates of potential GDP are based on the long-term natural rate. (CBO did not make explicit adjustments to the short-term natural rate for structural factors before the recent downturn.) The short-term natural rate incorporates structural factors that are temporarily boosting the natural rate beginning in 2008. The short-term natural rate is used to gauge the amount of current and projected slack in labor markets, which is a key input into CBO's projections of inflation.
Source: US. Congressional Budget Office
Release: Budget and Economic Outlook
US. Congressional Budget Office, Natural Rate of Unemployment (Short-Term) [NROUST], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/NROUST, April 28, 2016.