What is a Business Cycle?

This paper considers the question in its title from several angles. Part 1 looks at economic history and the development of thinking about business cycles - the popular meaning and economists' definitions and ideas. Part 2 reviews the lessons from business cycle chronologies and duration data, the concepts of periodicity of cycles and phases, and the apparent moderation of macroeconomic fluctuations in the second half of the 20th century. Part 3 compares the recent business cycles and growth cycles for several major industrialized, market-oriented countries. Part 4 discusses the role of endogenous cyclical variables, the outside shocks of various types, the systematic timing sequences, and the regularities of cyclical comovements and amplitudes. Understanding business cycles is aided by each of these models of analysis. Business cycles have varied greatly over the past 200 years in length, spread, and size. At the same time, they are distinguished by their recurrence, persistence, and pervasiveness. They make up a class of varied, complex, and evolving phenomena of both history and economic dynamics. Theories or models that try to reduce them to a single causal mechanism or shock are unlikely to succeed.