(Also called co-payment.) The specified portion (dollar amount or percentage) that Medicare, health insurance, or a service program may require a person to pay toward his or her medical bills or services.A cost-sharing requirement under a health insurance policy. It provides that the insured party will assume a portion or percentage of the costs of covered services. The health insurance policy provides that the insurer will reimburse a specified percentage of all, or certain specified, covered medical expenses in excess of any deductible amounts payable by the insured. The insured is then liable for the remainder of the costs until their maximum liability is reached.