Learn The Basics : Medicare Supplement
Medicare Supplement 
Sometimes referred to as Medigap, Medicare Supplement is an individual health insurance policy sold by private insurance companies. If you currently have Medicare Part A and Part B, you have the option to purchase a Medicare Supplement insurance policy. You are not able to use Medicare Supplement if you have a Medicare Advantage Plan or Medicaid.
Medicare Supplement insurance includes standardized plans, regulated by the government, that are designed to provide additional medical coverage for expenses not covered by Medicare, such as deductibles, coinsurance / co-payments, and your Part B monthly premiums.