Final answer:
Business cycles are the upward and downward changes in aggregate economic activity, as measured by GDP.
Step-by-step explanation:
The upward and downward changes in aggregate economic activity, as measured by GDP, are called business cycles. The business cycle refers to the short-term fluctuations of economic activity along its long-term growth trend. It consists of four phases: expansion or boom, contraction or recession, peak, and trough.