Define "coinsurance" in terms of health insurance.

Coinsurance is defined as the percentage of costs that a policyholder is responsible for paying after they have met their deductible. This arrangement typically applies to healthcare services and helps to share the financial responsibility between the insured individual and the insurance provider. Once the deductible has been satisfied, the insured will pay a predetermined percentage of the medical expenses, while the insurer covers the remaining portion. This means that if a policy has a coinsurance rate, for example, of 20%, the policyholder is responsible for 20% of the costs of covered services, while the insurer pays 80%. This system encourages policyholders to take a more active role in their healthcare decisions, as they are directly aware of their share of the costs involved.

The other definitions mentioned in the options refer to different concepts in health insurance. The maximum amount a policyholder pays per year is known as the out-of-pocket maximum, while a flat fee for every doctor's visit refers to a copayment. Cost sharing of premiums between an employer and employee is related to how premiums are divided and does not pertain to the coinsurance model.