Dictionary.net

Method of charging whereby a physician or other practitioner bills for each encounter or service rendered. In addition to physicians, other health care professionals are reimbursed via this mechanism. Fee-for-service plans contrast with salary, per capita, and prepayment systems, where the payment does not change with the number of services actually used or if none are used. (From Discursive Dictionary of Health Care, 1976)

When a customer asks a real estate agent to do an act for a fee.