Copay is a set expense for receiving care (for example, a doctor’s office visit) that must be paid per occurrence.
The patient pays the copay amount to the insurance carrier each time they receive a covered service. Copay amounts are set by the insurance carrier, and defined in an insurance policy.
Ultimately, the function of a copay is to discourage people from ordering too many/unnecessary medical services. By attaching some cost to otherwise subsidized medical care, people will have no advantage to accessing unlimited care to the point of wastefulness.
E.g.: An insurance carrier might charge a $25 copay/visit for a primary care visit, and a $50 copay/visit for a specialist visit.