An adjustable-rate mortgage is typically a loan through
a bank that has a varying interest rate.
For example, the mortgage could be called a 5/10 ARM which means a 5 year fixed rate followed by 10 years of a variable rate for a total of a 15 year mortgage.
The advantage of an adjustable-rate mortgage is that the early years tend to be at lower interest rates because they entice you to get it knowing that you will pay more interest later on.
