law: Negotiable Instruments Act, 1881 
  4. "Promissory note" 
  A "promissory note" is an instrument in writing (not being
  
  a bank-note 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.
  Illustrations 
  A signs instruments in the following terms:
  (a) "I promise to Pay B or order Rs.500".
  (b) "I acknowledge myself to be indebted to B in Rs.1,000,
  
  to be paid on demand, for value received."
  (c) "Mr B I.O.U Rs.1,000."
  (d) "I promise to pay B Rs. 500 and all other sums which
  
  shall be due to him."
  (e) "I promise to pay B Rs. 500 first deducting there out
  
  any money which he may owe me."
  (f) I promise to pay B Rs. 500 seven days after my marriage with
  
  C.
  (g) I promise to pay B Rs. 500 on D's death, provided D leaves
  
  me enough to pay that sum.
  (h) I promise to pay B Rs. 500 and to deliver to him my black
  
  horse on lst January next.
  The instruments respectively marked (a) and (b) are promissory
  
  notes. The instruments respectively marked (c), (d), (e), (f), (g) and (h) are
  
  not promissory notes.