The grading of a debt security with respect to the issuer's ability to meet interest and principal requirements in a timely manner. The three major rating services—Fitch, Moody's, and Standard & Poor's—use AAA as their highest rating and grade down through Bs and Cs. (D is used only by Fitch.) Debts rated AAA, AA, A, and BBB are considered investment-grade. Higher-rated bonds provide lower returns, the price an investor pays for greater safety. Also called security rating. Compare stock rating. See also interest coverage.