Health Maintenance Organization (HMO)
A. Health insurance plan that allows you to see any doctor, anytime.  Usually has a copayment and is cheaper if you use in-network doctors.
B. Health insurance plan that requires you to work with a primary care physician to manage you care.  Also  require you to use in-network services.
C. Health insurance plan that combines elements of HMO and PPO. You can use a Primary Care Physician or direct your own care.