1. Insurance carrier that reimburses the residual balance of the insurance claim after the primary insurance has paid its benefits. 2. When Medicare is the secondary payer (MSP), the primary insurance plan of a Medicare beneficiary must pay for any medical care or services first before Medicare is sent a claim. MSP may involve aged or disabled patients who are under group health plans and cases related to workers’ compensation, automobile accident, medical no-fault, and liability insurance. For a Medicare patient suffering from end-stage renal disease (ESRD), MSP is the payer for the first 30 months that the beneficiary is entitled to benefits.