A call is the option to buy the underlying stock at a predetermined price (the strike price) by a predetermined date (the expiry). The buyer of a call has the right to buy shares at the strike price until expiry. The seller of the call (also known as the call "writer") is the one with the obligation. Buying a call option entitles the buyer of the option the right to purchase the underlying futures contract at the strike price any time before the contract expires. This rarely happens, and there is not much benefit to doing this, so don’t get caught up in the formal definition of buying a call option. In finance, a short sale (also known as a short, shorting, or going short) is the assumption of a legal obligation to deliver to a buyer a financial asset that the seller does not own. If that obligation to deliver is immediate, that seller must borrow that asset at the very instant of that sale.