Definition of Negotiable Instruments: The NI Act defines negotiable instruments as written, transferable documents that promise payment of a specific amount of money, either on-demand or at a future date. Common types of negotiable instruments under the Act include: Promissory Notes: A written promise by one party to pay a specified sum of money to another party. Bills of Exchange: An order by one party to another to pay a specified sum to a third party. Cheques: An order to a bank to pay a specified sum to the person named in the cheque.