The exercise price, also called the strike price, of a future or option, is the price at which a derivative gives the right or obligation to buy or sell the underlying asset.
In most cases, the exercise price is simply a property of the security that is known from the outset and remains unchanged. Asian options and some exotic options have exercise prices that depend on the price of the underlying or its price history.
Traded options usually have a standardised set of exercise prices: an investor cannot find or write traded options at an arbitrary exercise price, but only at certain exercise prices (as well as expiry dates).