A valve on the main line that knocks municipal supply pressure down to a safe household range (usually 50–70 psi). Chronic high pressure hammers fixtures, appliances, and water heaters — a failed PRV shows up as banging pipes and shortened equipment life.
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.