A cost-sharing requirement under a health insurance policy which provides that the insured will assume a portion or percentage of the costs of covered services. Health care cost which the covered person is responsible for paying, according to a fixed percentage or amount. A policy provision frequently found in major medical insurance policies under which the insured individual and the insurer share hospital and medical expenses according to a specified ratio. A type of cost sharing where the insured party and insurer share payment of the approved charge for covered services in a specified ratio after payment of the deductible. Under Medicare Part B, the beneficiary pays coinsurance of 20 percent of allowed charges. Many HMOs provide 100% insurance (no coinsurance) for preventive care or routing care provided “in network”.