The amount charged a consumer of medical services when a service is partly covered by insurance.

A type of insurance policy where the insured pays a specified amount of out-of-pocket expenses for health-care services such as doctor visits and prescriptions drugs at the time the service is rendered, with the insurer paying the remaining costs. However, unlike coinsurance where the insured is required to pay a certain percentage of the covered costs, co-pay plans require the insured to pay a specified dollar amount.

http://www.answers.com/co-pay

according to your context, this is the terms usually referred to.