coinsurance

:  health insurance in which an insurer requires the insured to pay a fixed percentage of the cost of medical expenses after the deductible has been paid and with the insurer to pay the remaining expenses Coinsurance is not a new idea—Americans who grew up in the era before managed care will remember their parents paying 20% of the costs of office visits, tests, and prescriptions.—Thomas H. Lee et al., The New England Journal of Medicine, 24 July 2008

1 :  joint assumption of risk (as by two underwriters) with another

2 :  insurance (as fire insurance) in which the insured is obligated to maintain coverage on a risk at a stipulated percentage of its total value or in the event of loss to suffer a penalty in proportion to the deficiency