Delta tells you how much an option's price is expected to change when the underlying stock moves $1. A call with a delta of 0.50 gains roughly $0.50 for every $1 the stock rises; a put with a delta of -0.50 gains $0.50 when the stock falls $1. Delta ranges from 0 to 1 for calls and 0 to -1 for puts.

Delta as a probability

Traders also read delta as a rough estimate of the odds an option finishes in the money. A 0.30-delta call behaves loosely like a ~30% chance of expiring profitable. It is not exact, but it is a fast gut-check: a far out-of-the-money option with a 0.05 delta is a long shot, and it will move slowly until the stock comes to it.

Why delta changes - meet gamma

Delta is not fixed. As the stock moves, delta shifts, and the rate of that shift is gamma. Near expiration and near the strike, gamma spikes - delta can swing from 0.30 to 0.70 fast. That is what makes short-dated at-the-money options feel electric and dangerous at once. It is also the mechanic behind dealer gamma exposure and the gamma flip.

Delta is your speedometer. Gamma is how hard the accelerator is pressed.

Using delta in practice

Delta helps you size a position to the exposure you actually want. Higher-delta options track the stock closely and cost more; lower-delta options are cheaper but need a bigger move. Neither is "better" - they are different tools. What matters is choosing deliberately and pairing it with position sizing that matches the risk you can survive.