Medigap refers to various private supplemental health insurance

Health insurance

Health insurance is insurance against the risk of incurring medical expenses among individuals. By estimating the overall risk of health care expenses among a targeted group, an insurer can develop a routine finance structure, such as a monthly premium or payroll tax, to ensure that money is...

 plans sold to Medicare

Medicare (United States)

Medicare is a social insurance program administered by the United States government, providing health insurance coverage to people who are aged 65 and over; to those who are under 65 and are permanently physically disabled or who have a congenital physical disability; or to those who meet other...

 beneficiaries in the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 that provide coverage for medical expenses not or only partially covered by Medicare

Medicare (United States)

Medicare is a social insurance program administered by the United States government, providing health insurance coverage to people who are aged 65 and over; to those who are under 65 and are permanently physically disabled or who have a congenital physical disability; or to those who meet other...

. 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. As of 2006, 18% of Medicare beneficiaries were covered by a Medigap policy.
A person must be enrolled in part A and B of Medicare before they can enroll in a Medigap plan.