A percentage of the medical costs to be shared by the insurer and the insured after the deductible has been met. In an “80/20 to $5000” plan, the insured will pay his deductible (e.g. $500) and also 20 percent (the co-insurance) of the first $5000 of medical costs. Then the insurance company will cover everything. Note that Co-insurance is not the same as Co-pay.