Co-insurance is an insurance-related term that describes a splitting or spreading of risk among multiple parties.
coinsurance n : insurance issued jointly by two or more underwriters
Coinsurance \Co`in*sur"ance\, n. [Co- + insurance.] Insurance jointly with another or others; specif., that system of fire insurance in which the insurer is treated as insuring himself to the extent of that part of the risk not covered by his policy, so that any loss is apportioned between him and the insurance company on the principle of average, as in marine insurance or between other insurers. [Webster 1913 Suppl.]
In the United States, copayment or copay is a payment defined in the insurance policy and paid by the insured person each time a medical service is accessed.
A copayment often goes on indefinitely. For example, if a Doctor's office visit costs $125.00 per visit and your co-pay is $15.00 per visit that $15.00 per visit fee will be permanent.