the percentage – of the healthcare coverage – that the insured must pay (despite having health insurance)
EXAMPLE:
John Doe has a health insurance policy. His policy has a 20% coinsurance. On February 1, 20XX, Mr. Doe suffers an ankle injury. So, he goes to the hospital. The hospital bills him $6,000 (for providing healthcare). Due to the coinsurance on his policy, John Doe must pay the first $1,200 (which is 20% of $6,000). His health insurance policy, on the other hand, pays the rest of the bill (ie, $4,800).
compare with copay; deductible.