Strike price
In the context of derivatives the strike price of an option is a key variable in a financial contract between two parties. Typically an option has positive monetary value if an underlying financial instrument (e.g. a stock price, interest rate or inflation rate) has a value above (or below depending on the particular type of contract, but not both) the strike price.
In the context of a call option, the payoff is 
where S is the final of the underlying, K is the strike and where
For a digital option 
where 
is the indicator function.
