The valve that stops all water to the house — usually where the service line enters, near the meter. Every adult in the home should know where it is; a seized shutoff during a burst pipe turns a mess into a disaster.
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.