The published cost of money that serves as the minimum basis for determining the interest rate for an adjustable rate mortgage. Among the commonly used indices are the Prime Rate (Prime), the London Interbank Offering Rate (LIBOR), the Cost of Funds (COF) and the 1 year Treasury Bill (1 year T). The particular index is generally, though not always, selected based on how often an interest rate is supposed to adjust. Loans that allow monthly interest rate adjustments commonly use the Prime Rate.