Gamma
The rate of change of delta with respect to a $1 change in the underlying asset price.
Explanation
Gamma is highest for ATM options near expiration and measures how quickly delta changes. High gamma means delta is very responsive to price moves, creating larger P&L swings. Long options have positive gamma (beneficial); short options have negative gamma (risky near expiration). Gamma risk is a key concern for options sellers.
Example
A call with delta 0.50 and gamma 0.05 will have a delta of 0.55 after the underlying moves up $1.