Since options depend on a number of input factors, they must change in value when an input factor changes in value. The strike price as well as the maturity date are deterministic; i.e. once they are set, they do not change any more. The other input factors, price of the underlying asset, volatility, interest rate and net yield, can change over time. For  example, the impact of a change in volatility on the option price, all else being equal, is the sensitivity of the option price to volatility.

Most important and obvious, the option price is sensitive to a movement in the underlying asset. The change in the option value divided by the change in the underlying asset is called the delta. The delta of a call option is between zero and one while the delta of a put option is between minus one and zero. The delta is an important parameter with regard to the replicating portfolio.

Since it measures the price change of an option due to a price change in the underlying asset, the delta actually is the exact number of underlying assets that must be held in the replicating portfolio. Somebody who intends to hedge an option should therefore hold a delta amount of underlying assets. This procedure is called delta hedging.