What is a negative amortization loan?

In finance, negative amortization, also known as NegAmMort, is an amortization method in which the borrower pays back less than the full amount of interest owed to the lender each month. The shorted amount is then added to the total amount owed to the lender. Such a practice would have to be agreed upon before shorting the payment so as to avoid default on payment. Also known as deferred interest or Graduated Payment Mortgage (GPM).