In finance, the strike price (or exercise price) of an option is the fixed price at which the owner of the option can buy (in the case of a call), or sell (in the case of a put), the underlying security or commodity.
The strike price is a key variable in a derivatives contract between two parties. Where the contract requires delivery of the underlying instrument, the trade will be at the strike price, regardless of the spot price (market price) of the underlying instrument at that time.
For example, an IBM May Call a strike price of $50 a share. When the option is exercised, the owner of the option will buy 100 shares of IBM stock for $50 per share.
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Moneyness
Moneyness is the value of a financial contract if the contract settlement is financial. More specifically, it is the difference between the strike price of the option and the current trading price of its underlying security.
In options trading, terms such as inthemoney, atthemoney and outofthemoney describe the moneyness of options.
 A call option is inthemoney if the strike price is below the market price of the underlying stock.
 A put option is inthemoney if the strike price is above the market price of the underlying stock.
 A call or put option is atthemoney if the stock price and the exercise price are the same (or close).
 A call option is outofthemoney if the strike price is above the market price of the underlying stock.
 A put option is outofthemoney if the strike price is below the market price of the underlying stock.
Mathematical formula
A call option has positive monetary value at expiration when the underlying has a spot price (S) above the strike price (K). Since the option will not be exercised unless it is inthemoney, the payoff for a call option is
also written as
where
A put option has positive monetary value at expiration when the underlying has a spot price below the strike price; it is "outthemoney" otherwise, and will not be exercised. The payoff is therefore:
or
For a digital option payoff is , where is the indicator function.
See also
References
 McMillan, Lawrence G. (2002). Options as a Strategic Investment (4th ed. ed.). New York : New York Institute of Finance. ISBN 0735201978.
