Promissory note defines in Section 4 of the Negotiable Instruments Act,1881 which says it's an agreement where one person promises to pay back money to another person.

Section 4 of the Negotiable Instruments Act, 1881 defines a “Promissory Note.”
It states:
A promissory note is an instrument in writing (not being a banknote or a currency note), containing an unconditional undertaking, signed by the maker, to pay a certain sum of money only to, or to the order of, a certain person, or to the bearer of the instrument.