How far an economy’s current output is below what it would be at full capacity. On average, inflation rises when output is above potential and falls when output is below potential. However, in the short run, the relationship between inflation and the output gap can deviate from the longer-term pattern and can thus be misleading. Alas for policymakers – because nobody really knows what an economy’s potential output is, the size and even the direction of the output gap can easily be misdiagnosed, which can contribute to serious errors in macroeconomic policy.