Peak
Plain definition
A peak is the highest point of the business cycle — the moment when expanding economic activity stops rising and begins to turn downward. It marks the end of an expansion and the start of a contraction.
In economics
Peaks are dated by the NBER months after they occur, since only hindsight reveals the turning point. Economists watch for warning signs of an approaching peak: inverted yield curves, rising inventories, slowing job growth, and overheated asset prices. Monetary policy often tightens near peaks as central banks try to cool inflation without triggering the downturn.
Example
The NBER dated the pre-pandemic U.S. expansion's peak to February 2020. Employment, output, and retail sales were at record highs — yet by March the COVID-19 shock had pushed the economy into contraction. The peak marked the exact month the expansion ended, even though no one called it in real time.
A business-cycle peak is not the same as a stock market peak: equities often top out before or after the economic peak, since they price in expectations. Also, 'peak' does not mean the economy was healthy — late expansions frequently carry imbalances (bubbles, debt) that make the coming contraction worse.