What is the term for the alternating periods of economic expansion and contraction?

The term for the alternating periods of economic expansion and contraction is the business cycle.

A business cycle describes recurring fluctuations in economy-wide activity. During an expansion, production, employment, incomes, sales, and investment generally rise. A contraction, often called a recession when it is broad and sustained, brings widespread declines in those measures. The pattern is recurring but not perfectly regular or mechanically predictable.

The modern statistical study of business cycles was shaped by Arthur F. Burns and Wesley C. Mitchell, whose 1946 book Measuring Business Cycles defined cycles as fluctuations in aggregate economic activity. In the United States, the National Bureau of Economic Research dates expansions from a trough to a peak and recessions from a peak to a trough.

A common mix-up is treating the business cycle as a fixed timetable. Episodes vary in length and severity, and economists usually distinguish them from long-term growth trends. The phrase economic cycle can be understandable in casual speech, but business cycle is the standard canonical term for this macroeconomic concept.