The Negotiable Instrument Act of 1881 defines and regulates negotiable instruments like promissory notes, bills of exchange, and cheques in India. It defines a negotiable instrument as a document that can be transferred to another party, and outlines characteristics like being in writing, containing an unconditional undertaking to pay a certain sum of money, and enabling the holder to receive payment. The Act also specifies types of negotiable instruments and endorsements that can be made for the purpose of negotiation.