BUYER'S OPTION, a written contract under the terms of which a buyer of securities need not accept delivery until the end of a specified time but may demand delivery at any time within the limit upon one day's notice. On the New York Stock Exchange a buyer's option must be for a term of not less than 4 nor more than 6o days. The buyer, unless the contract is "flat" (without interest), pays the seller interest on the price of the security at the legal rate up to the date of delivery.