Which instrument is defined by section 5 of the Negotiable Instruments Act, 1881?

Section 5 of the Negotiable Instruments Act, 1881 defines a bill of exchange as a signed written instrument containing an unconditional order directing a certain person to pay a certain sum of money only to, or to the order of, a certain person or to the bearer. The promissory note is defined in section 4, the cheque in section 6 and the negotiable instrument in section 13.