The buried drain line carrying everything from your house to the public sewer main — and the homeowner's responsibility, not the utility's. Root intrusion and collapsed clay laterals are the classic five-figure surprise in pre-1980 homes; a camera inspection sees it before you buy.
Plumbing terminology in MD, DC, and Northern Virginia maps to older supply lines, basement sump pumps, water pressure regulation, and the licensing (MD PLB & GFC, VA, DC) that real plumbing work requires.