Bill of exchange: written unconditional order to pay specified payee or bearer; timing and interest not make it conditional. A bill of exchange is a written instrument containing an unconditional order, signed by the maker, directing a specified person to pay a certain sum to a particular payee or bearer. Time expressions tied to events certain to occur do not make the order conditional. The sum may include future interest or depend on an indicated rate of exchange, and a clause accelerating the balance on default is permitted; a misnamed or descriptively designated payee may still be a certain person.