译文（字数 4995）

摘 要

商业周期（或经济周期）在学术上指的是在遵循自由企业经济规则的条件下，经济在生产、贸易和经济活动上呈现的年或月的大幅度波动。

商业周期的大小取决于GDP（国内生产总值）水平的高低，具体指的是经济活动在长期增长趋势这一阶段的扩张和收缩。

围绕长期增长趋势发生的这些波动，并通常涉及在经济较快增长期间（膨胀或繁荣），和时间的相对停滞或下降期间（收缩或衰退）。

商业周期通常是通过考虑国内实际生产总值增长率来判断。虽然被称为周期，但是这些经济活动的波动规律并不是按照一个机械的或可预见的周期模式去变动。

外文原文（字符数 18179）

Introduction

The term business cycle (or economic cycle) refers to economy-wide fluctuations in production, trade and economic activity in general over several months or years in an economy organized on free-enterprise principles.

The business cycle is the upward and downward movements of levels of GDP (gross domestic product) and refers to the period of expansions and contractions in the level of economic activities (business fluctuations) around its long-term growth trend. 

These fluctuations occur around a long-term growth trend, and typically involve shifts over time between periods of relatively rapid economic growth (an expansion or boom), and periods of relative stagnation or decline (a contraction or recession).

Business cycles are usually measured by considering the growth rate of real gross domestic product. Despite being termed cycles, these fluctuations in economic activity do not follow a mechanical or predictable periodic pattern.