Mean reversion
The tendency of a stretched price or ratio to move back toward its historical average — the mirror image of momentum, and often at odds with it.
Mean reversion is the idea that extremes tend not to last: a price far above or below its typical level often drifts back toward the average. Approaches built on it buy weakness and sell strength — the opposite instinct to trend following.
The difficulty is timing: cheap can get cheaper and stretched can stretch further, for longer than anyone expects. Momentum and mean reversion both appear in markets, on different horizons, which is why no single pattern is treated as reliable on its own.