Define "coinsurance."

Coinsurance is a cost-sharing arrangement commonly found in health insurance plans where the insured is responsible for paying a certain percentage of the costs of covered healthcare services after the deductible has been met. For example, if a health insurance plan has a coinsurance rate of 20%, the insured would pay 20% of the covered medical expenses, while the insurance company would pay the remaining 80%. This arrangement helps to share the costs between the insurer and the insured and ensures that the insured has a financial stake in the healthcare services they use, potentially preventing over-utilization.

The other options reflect different concepts related to healthcare expenses. A fixed amount paid for a specific healthcare service describes a copayment (or copay), which is a predetermined fee. A flat fee paid for every medical service mirrors the idea of copayments as well, and does not involve percentage sharing. Lastly, the definition of total out-of-pocket costs pertains to the overall expenses incurred by a policyholder, which includes deductibles, copayments, and coinsurance, rather than specifically referring to the percentage-based cost-sharing mechanism of coinsurance itself.