Gamma
The rate of change of delta per $1 move in the underlying. Long options have positive gamma (delta moves in your favor); short options have negative gamma (exposure grows against you). Gamma is largest at the money and near expiration.
Gamma is why short-option positions feel fine until they suddenly do not: as the stock approaches a short strike, the position picks up delta in the wrong direction exactly when moves are hurting.
The gamma–theta trade-off is the central bargain of options: positive gamma is paid for with daily decay, negative gamma is compensated with daily income. Neither is free; the question is always the price.
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