Final answer:
Co-insurance is the term used when an insurance policy covers a certain percentage of a cost, and the patient is responsible for the remaining percentage, such as an 80-20 split where insurance pays 80% and the patient pays 20% of the cost.
Step-by-step explanation:
When a patient's insurance covers a certain percentage of the cost of a prescription, and the patient is responsible for the remaining percentage, this payment structure is referred to as Co-insurance. Specifically, if the insurance covers 80% of the cost, and the patient pays the other 20%, the patient is participating in a co-insurance agreement. This is distinct from a Co-payment (or co-pay), which is a fixed amount the patient pays regardless of the total cost; a Deductible, which is an amount the patient must pay out-of-pocket before insurance starts to cover expenses; and a Premium, which is the regular amount paid for the insurance policy itself.