from The American Heritage® Dictionary of the English Language, 4th Edition
- n. Private health insurance designed to supplement the coverage provided under governmental programs such as Medicare.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- proper n. A type of medical insurance for expenses not covered under Medicare.
Medi(care) or Medi(caid) + gap.(American Heritage® Dictionary of the English Language, Fourth Edition)
Blend of Medicare and gap (Wiktionary)