Co-insurance is a policy provision under which you and the insurance company share the total cost of covered medical services after the deductible has been met set as fixed percentages.

For illustrative purposes, a particular plan may provide 75/25% coinsurance (after a $2,000 deductible has been met) up to a specified amount called the coinsurance out of pocket maximum (in this case $3,000).

This means that you'll pay the the first $2,000 of covered expenses subject to the deductible. Then, you'll pay 25% of all covered expenses up to the point that you've paid $3,000, (while the insurance company pays 75%). After that, the plan typically pays 100% of covered charges for the remainder of the year on an in network basis, up to the policy maximum.

Please note that coinsurance percentages and coinsurance out of pocket maximums differ out of network with you covering a greater portion of the cost overall. Once met, most policies typically only cover the maximum allowable amount of the usual and customary rate (UCR). You can be balanced billed for the difference. See "What is the usual and customary rate (UCR)?"