How much an option's price moves per $1 move in the underlying — the exposure dealers neutralize.
Delta measures how much an option's price changes for a $1 move in the underlying, and doubles as the option's directional exposure. It is precisely the quantity market makers must neutralize to stay delta neutral, making it the unit of the hedging flow this site tracks.
More terms: Gamma wall · Delta neutral · Gamma · 0DTE options · Dealer hedging · Call wall