An insurance arrangement where both the insured person and the insurance company share the cost of losses; also, insurance shared between multiple parties.
From co- (together) + insurance (from Old French, assurance). Became a formal insurance term in the 20th century to describe cost-sharing arrangements.
Your health insurance's copay is coinsurance—you pay part, the insurance company pays part. It's designed to make patients think twice before running to the doctor for every tiny thing, while still protecting them from bankruptcy.
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