经济周期(business cycle)
经济活动比率（按就业、价格和生产的水平计量）周期性的波动。经济学家长期以来一直为繁荣时期之後为什麽总是出现经济危机（股市崩盘、银行倒闭、失业等）而争论不休。有人提出每八～十年为一个周期，康得拉季耶夫则主张更长的周期。除了偶然的冲击（战争、技术变化等等）之外，投资和消费是影响经济活动水平的主要因素。增加投资，譬如建造工厂，除原有开支本身之外还要创造其他的收入，因为建厂的工人们还得用掉他们的工资。相反地，如果消费需求增加了，为了满足需求最终还得建造新的工厂。最终经济总会到达它容量的极致，这时候很少自由资本，没有新的需求，因此程序又颠倒过来，结果产生紧缩。农业市场的自然波动、时尚风潮等心理因素以及货币供给（money supply）的变化，是投资或消费方面造成这些变化的起因。自第二次世界大战以来，政府以货币政策做为调节经济周期的手段：在经济呆滞时期起刺激作用以避免停滞性通货膨胀或经济萧条，在经济扩张时期起抑制作用。
English version：
business cycle
Periodic fluctuation in the rate of economic activity, as measured by levels of employment, prices, and production. Economists have long debated why periods of prosperity are eventually followed by economic crises (stock-market crashes, bankruptcies, unemployment, etc.). Some have identified recurring eight-to-ten-year cycles in market economies; longer cycles have also been proposed, notably by Nikolay Kondratev. Apart from random shocks to the economy, such as wars and technological changes, the main influences on the level of economic activity are investment and consumption. An increase in investment, as when a factory is built, leads to consumption because the workers employed to build the factory have wages to spend. Conversely, increases in consumer demand cause new factories to be built to satisfy the demand. Eventually the economy reaches its full capacity, and, with little free capital and no new demand, the process reverses itself and contraction ensues. Natural fluctuations in agricultural markets, psychological factors such as a bandwagon mentality, and changes in the money supply have all been proposed as explanations for initial changes in investment and consumption. Since World War II, government monetary policy has aimed at moderating the business cycle, preventing the extremes of inflation and depression by stimulating the economy in slack times and restraining it during expansions.