typically, a minority would be anyone that is not white. It is a racial/ethnic distinction. So far as I know, religion and sexual orientation are not used to determine minority status.
An underrepresented minority is something different. That is a minority whose racial or ethnic group makes up a smaller proportion of physicians than it should. By "should" I mean the proportion it would if every group was represented in the physician population in equal proportion to the general population. For example, Asian-Americans are not URMs, but African Americans are.