What is the difference between a copayment and coinsurance?

A copayment (copay) is a fixed dollar amount the insured pays for a specific covered service — for example, $25 for a doctor visit. Coinsurance is a percentage of the cost of a covered service that the insured pays, usually after meeting the deductible — for example, the plan pays 80% and the insured pays 20% coinsurance. Both are forms of cost-sharing that divide expenses between the insured and the insurer. The out-of-pocket maximum caps the total an insured pays in a period. Distinguishing the fixed-dollar copay from the percentage-based coinsurance is a commonly tested cost-sharing concept.