When you purchase a call option, you are paying for the choice to buy shares of an underlying stock at a specified price by a certain date. (creditdonkey.com)
A call option is defined by the underlying stock, price of the underlying stock at which the option can be exercised — strike price — and the expiration date. (pocketsense.com)
For call options, the options holder can demand that the options seller sell shares of the underlying stock at the strike price. (investopedia.com)