The document discusses the business cycle, which refers to recurring periods of economic expansion and contraction. It describes the key phases of a business cycle as peak, recession, trough, and recovery. During a peak, economic activity is at its highest, while a recession involves declining output, income and employment. Theories on what causes business cycles include influences from climate, psychology, monetary policy, over-investment, and innovation. Models can view the business cycle as stemming from external/exogenous factors or internal/endogenous factors within the economic system.