A put option (sometimes simply calle a "put") is a financial contract between two parties, the seller (writer) and the buyer of the option. The put allows the buyer the right but not the obligation to sell a commodity or financial instrument (the underlying instrument) to the writer (seller) of the option at a certain time for a certain price (the strike price). The writer (seller) has the obligation to purchase the underlying asset at that strike price, if the buyer exercises the option.