Co-insurance is the percentage of a covered health care service you have to pay after you've paid your deductible. It is more commonly used in the US.
For example, if your plan only covers 80% of the services, then you are expected to co-insure (pay) the remaining 20%. If you haven't met your deductible, then you will have to pay the full amount of the medical service.
Good plans usually limit the co-insurance to a certain out-of-pocket sum.