A strike with very large options open interest where dealer gamma concentrates and price tends to pin.
A gamma wall is a strike — or cluster of strikes — that holds unusually large options open interest, so dealer gamma is concentrated there. Because market makers must hedge that exposure, price tends to be drawn toward the wall and stall (or "pin") near it. Watching the wall's rate of change matters as much as its location: a wall that grows quickly forces fresh hedging that can move the market.
More terms: Delta neutral · Gamma · 0DTE options · Dealer hedging · Call wall · Put wall