"The AAMC definition of underrepresented in medicine is: "Underrepresented in medicine means those racial and ethnic populations that are underrepresented in the medical profession relative to their numbers in the general population.""
I think this page describes it pretty well https://www.aamc.org/initiatives/urm/. I think it is basically Mexican-Americans, Puerto Ricans, African-Americans, and Native Americans.
You will be happy to learn that Arab americans are well represented in the physician workforce. Arab IMG's alone comprise 1.5% of practicing physicians. This is a disproportionately large representation for their presence in the US population. http://www.naama.com/about/arab-american-physicians/