The stage of the business or market cycle from the end of a period of growth (peak) into declining activity and contraction until it hits its ultimate cyclical bottom (trough). 
This is the change in a data measure - e.g., an asset price or economic statistic - from its highest point to its lowest point. The term is often used to refer to declines during a cyclical downturn.
The peak-to-trough decline in gross domestic product during a recession is the total contraction in output from the point when the recession began to the point when the economy started to grow again. Also peak-to-trough falls in house prices, commercial property prices, industrial output etc.