The index for an adjustable-rate mortgage (ARM) is tied to a published benchmark (such as LIBOR, SOFR, or U.S. Treasury securities). Index rates fluctuate over time as the general level of interest rates in the market changes.

“The index is a published interest rate to which the interest rate on an ARM is tied. The index rate changes over time, usually in line with general market rates.”

— CFPB, Consumer Handbook on Adjustable-Rate Mortgages (CHARM)