A Medicare Supplement  (or Medigap ) refers to various private supplemental health insurance plans sold to Medicare beneficiaries in the United States that provide coverage for medical expenses not or only partially covered by Medicare.  Medigap's name is derived from the notion that it exists to cover the difference or "gap" between the expenses reimbursed by Medicare and the total amount charged. 

A person must be enrolled in part A and B of Medicare before they can enroll in a Medigap plan. During the open enrollment period a person may obtain a Medigap plan on a guaranteed issue basis (i.e. no medical screening required). Outside of open enrollment, the issuing insurance company may require medical screening and may obtain an attending physician's statement if necessary. Medigap insurance is not compatible with other forms of private Medicare coverage, such as a Medicare Advantage plan. Medigap, offerings have been standardized by the Centers for Medicare and Medicaid Services (CMS) into twelve different plans, labeled A through L, sold and administered by private companies. Each Medigap plan offers a different combination of benefits