Gamma
How fast an option’s delta changes as the stock moves.
Gamma is the acceleration behind delta. When gamma is high, small moves in the stock cause big swings in an option’s delta and price, which is why near-the-money options close to expiration feel so twitchy. Zero-day (0DTE) options are famous for extreme gamma. Market makers hedging their gamma is a big driver of index price action.
For example
A near-the-money option on expiration day can flip from barely in the money to deeply in the money in minutes, because its gamma is enormous.
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