Amortising swap is usually an interest rate swap

Interest rate swap

An interest rate swap is a popular and highly liquid financial derivative instrument in which two parties agree to exchange interest rate cash flows, based on a specified notional amount from a fixed rate to a floating rate or from one floating rate to another...

 in which the notional principal for the interest payments declines during the life of the swap, perhaps at a rate tied to the prepayment of a mortgage or to an interest rate benchmark such as the London Interbank Offered Rate

London Interbank Offered Rate

The LIBOR rate is the average interest rate that leading banks in London charge when lending to other banks. It is an acronym for London Interbank Offered Rate Banks borrow money for one day, one month, two months, six months, one year etc. and they pay interest to their lenders based on...

 (Libor). If the swap allows for uncertain contingent ups and downs in the notional principal, it is called a "roller-coaster swap". The opposite of the accreting swap.