The term for the lowest unemployment rate sustainable without causing inflation to accelerate is NAIRU.
NAIRU stands for “non-accelerating inflation rate of unemployment.” It describes a theoretical unemployment level at which inflation is stable; pushing unemployment below it is expected to create accelerating price growth as labor markets tighten and wages rise faster.
The idea grew from economist Milton Friedman’s 1968 discussion of the “natural rate” of unemployment and was developed further in 1970s macroeconomics. NAIRU is not a directly observable number: economists estimate it using models, and the estimate can change with productivity, demographics, labor-market institutions, and the structure of the economy.
A common mix-up is treating NAIRU as identical to full employment. Full employment can still include frictional and structural unemployment, while NAIRU specifically focuses on the inflation consequences of unemployment. The natural rate of unemployment is closely related and is sometimes used as a synonym, but NAIRU is the canonical answer when the question explicitly mentions accelerating inflation.