A call option, often simply labeled a "call", is a financial contract between two parties, the buyer and the seller of this type of option. The buyer of the call option has the right, but not the obligation to buy an agreed quantity of a particular commodity or financial instrument (the underlying) from the seller of the option at a certain time (the expiration date) for a certain price (the strike price). The seller (or "writer") is obligated to sell the commodity or financial instrument should the buyer so decide. The buyer pays a fee (called a premium) for this right.

The buyer of a call option purchases it in the hope that the price of the underlying instrument will rise in the future. The seller of the option either expects that it will not, or is willing to give up some of the upside (profit) from a price rise in return for the premium (paid immediately) and retaining the opportunity to make a gain up to the strike price (see below for examples).

Call options are most profitable for the buyer when the underlying instrument moves up, making the price of the underlying instrument closer to, or above, the strike price. The call buyer believes it's likely the price of the underlying asset will rise by the exercise date. The risk is limited to the premium. The profit for the buyer can be very large, and is limited by how high the underlying instrument's spot price rises. When the price of the underlying instrument surpasses the strike price, the option is said to be "in the money".