An adjustable rate mortgage, or ARM for short, is a mortgage loan which does not have a fixed interest rate throughout its term. With an adjustable rate mortgage (ARM) the interest rate is subject to periodic adjustment. The rate adjustment may be based on any time period (daily, monthly, quarterly, semi-annually, annually, etc.) and the adjusted rate is typically expressed as a spread or margin over a defined index rate. Typical index rates include LIBOR, Prime Rate, and the 30-Day US Treasury rate.