Which instrument is defined by section 4 of the Negotiable Instruments Act, 1881?

Section 4 of the Negotiable Instruments Act, 1881 defines a promissory note as a signed written instrument, not a bank note or currency note, containing an unconditional undertaking to pay a certain sum of money to a certain person or to his order or to bearer. The bill of exchange is defined in section 5, the cheque in section 6 and the negotiable instrument in section 13.