Amortization means that monthly payments are large enough to pay the interest and reduce the principal on your mortgage. Negative amortization occurs when the monthly payments do not cover all of the interest cost. The interest cost that isn't covered is added to the unpaid principal balance. This means that even after making many payments, you could owe more than you did at the beginning of the loan. Negative amortization can occur when an ARM has a payment cap that results in monthly payments not high enough to cover the interest due.
A situation in which a borrower is paying less interest than what is actually being charged for a mortgage loan. The unpaid interest is added to the loan's principal. The borrower may end up owing more than the original amount of the mortgage.
Source: Ginnie Mae