The U-shaped bend under every fixture that holds a plug of water to block sewer gas from entering the home. A dry trap (common in rarely-used basement floor drains) is the usual culprit behind a mystery sewer smell.
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.