The portion of the original loan amount represented by the periodic, equal debt service payments. The decimal factor that is applied to the original mortgage or deed-of-trust amount to find the level-payment, full-amortized mortgage debt service, payable each equal time period. This payment factor, usually quoted in decimal-fraction form, covers the repayment of principal and payment of interest each mortgage-payment period. There are many combinations of interest rate, loan term to maturity, and frequency of loan payment per year that may be represented by a single loan constant.