The business cycle is a periodic but irregular up and down movement in economic activity measured by fluctuations in real GDP and other macroeconomic variables. A business cycle is not a regular, predictable, or repeating phenomenon like the swing of the pendulum of a clock. Its timing is random and usually unpredictable. 
A business cycle is identified as a sequence of four phases:

·         Contraction: A slowdown in the pace of economic activity

·         The lower turning point of a business cycle, where a contraction turns into an expansion

·         Expansion: A speedup in the pace of economic activity

·         Peak: The upper turning of a business cycle

source - About.com