A managed care plan that combines the function of insurer and provider to give members comprehensive health care from a network of affiliated providers. Enrollees typically pay limited copayments and are usually required to select a primary care physician through whom all care must be coordinated. HMOs generally will not reimburse all costs for services obtained from a non-network provider or without a primary care physician’s referral. HMOs often emphasize prevention and careful assessment of medical necessity. (Also see group-model HMO, network-model HMO and staff-model HMO.)