An adjustable rate mortgage, commonly referred to as an ARM, is a loan type that allows the lender to adjust the interest rate during the term of the loan. Generally, these changes are determined by a margin and an index so that the interest rate changes, up or down, are based on market conditions at the time of the change. Most often these interest rate changes are limited by a rate change cap and a lifetime cap. If you apply for an adjustable rate mortgage, the lender is required to provide you with an ARM Program Disclosure which spells out the terms of the loan.