Expansion, peak, recession, and trough are all periods of the

The Economic Cycle.
An economy when graphed will essentially look like a wave. It may not be perfect, as there will be different size expansions and contractions/recessions.
Expansions is when the market is doing well or correcting from a recession. The Peak is the highest point that specific economic period can achieve. From there, the market will enter a Recession or Contraction where it goes down. It’s Trough is the lowest point that the contraction will reach.