The strike price, or exercise price, is stated in all option contracts between two parties, for example in a stock option contract.
It is the price at which the owner of a call option can purchase (or the owner of a put option can sell) the underlying asset to the writer of the option. For example:
If you have a Microsoft August $70 call, you have the right but not the obligation to purchase 100 shares of Microsoft for a price of $70 – that is the strike price… Strikes are usually available in $5 increments and in $2.50 increments for stocks under $25.
Bill Johnson with James DiGeorgia and David Nichols, An Investor’s Guide to Understanding and Mastering Options Trading
In the case of stock options, a call option is in-the-money if the market price of the stock is higher than the strike price. A put option is in-the-money if the market price of the stock is lower than the strike price.