prime rate 
noun chiefly North American
The lowest rate of interest at which money may be borrowed commercially: the banks have cut their prime rate by 4 percentage points...
- The average rate for the credit lines is the prime rate - the interest rate banks charge their most creditworthy customers.
- The interest rate would be 8% (the prime rate at that time) with no prepayment penalty clause.
- Some short-term home-equity loans are tied to the prime rate; longer-term rates may be a point or two higher than your existing mortgage.