The rate of change of an option's delta as the underlying moves.
Gamma is the rate of change of an option's delta for a $1 move in the underlying. High gamma means a dealer's hedge has to be adjusted rapidly and repeatedly, which amplifies their buying and selling near key strikes. Gamma is highest near the money and near expiration.
More terms: Gamma wall · Delta neutral · 0DTE options · Dealer hedging · Call wall · Put wall