Coinsurance in insurance, is the splitting or spreading of risk among multiple parties. In the U.S. insurance market, coinsurance is the joint assumption of risk between the insurer and the insured. In health insurance, coinsurance is sometimes used synonymously with copayment, but copayment is really fixed while coinsurance is a percentage that the insurer pays after the insurance policy's deductible is exceeded up to the policy's stop loss.