Delta
The option Greek that measures how much the contract price changes when the underlying moves by one unit — essential when hedging.
Definition
Delta is the Greek that shows how much the theoretical price of an option changes when the underlying instrument moves by one unit. It is usually positive for calls and negative for puts; values closer to plus or minus one mean the option behaves more like a position in the underlying itself.
Example
You hold a call on a GPW-listed stock with a delta of 0.40. If the share price rises by 1 PLN, the pricing model suggests the option premium gains about 0.40 PLN (all else held equal). A short put with a delta of −0.30 behaves in the opposite direction on the same upward move.
Anti-example / common mistake
Delta is not a guarantee of profit and it is not a constant for as long as you hold the position — it changes with price, time to expiry and volatility. Do not confuse delta with the probability of a certain win, or with the broker leverage on a CFD.