Business cycles are a type of fluctuation found in the aggregate activity of nations that organize their work mainly in business enterprises: a cycle consists of expansions occurring at about the same time in many economic activities, followed by similarly general recessions, contractions, and revivals which merge into the expansion phase of the next cycle; this sequence of changes is recurrent but not periodic; in duration business cycles vary from more than one year to ten or twelve years; they are not divisible into shorter cycles of similar character with amplitudes approximating their own.

Arthur F. Burns and George W. Mitchell (1946). Measuring business cycles. New York: National Bureau of Economic Research. p. 3; Cited in: Robert J. Gordon, ed. The American Business Cycle: Continuity and Change, 1986. p. 2