"Parity" means "equal to". In health care, parity means that mental health and addiction treatment services are delivered at the same level, frequency and availability as medical and surgical services.
Mental Health Parity and Medical Necessity in California: Frequently Asked Questions, The Kennedy Forum
Generally Accepted Standards of Behavioral Health Care (including "Wit v. United Behavioral Health" - 2019 Landmark Ruling), The Kennedy Forum
Disparities in Network Use and Reimbursement Rates, Overview of Milliman Report and Findings, Dec. 2017