Health Maintenance Organization. A form of health insurance combining a range of coverages in a group basis. A group of doctors and other medical professionals offer care through the HMO for a flat monthly rate with no deductibles. However, only visits to professionals within the HMO network are covered by the policy. All visits, prescriptions and other care must be cleared by the HMO in order to be covered. A primary physician within the HMO handles referrals.