Mortgage interest refers to the amount of interest that a borrower is charged for taking out a loan to purchase real estate. It is typically calculated as a percentage of the total loan amount and paid over time along with the principal balance of the loan. The interest rate on a mortgage can fluctuate based on market conditions, but it is generally fixed for the life of the loan. Mortgage interest payments are often tax-deductible, which can help to offset some of the cost for homeowners.