Zinc-coated steel supply piping common before the 1960s. It corrodes shut from the inside over decades — falling pressure and rusty morning water are its signature — and it's a whole-system replacement candidate rather than a patch-forever material.
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.