A one-way valve — on a sump discharge line it stops ejected water from draining back into the pit. When it fails, the pump re-pumps the same water on a loop until it burns out.
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.