A sacrificial metal rod inside a tank water heater that corrodes on purpose so the steel tank doesn't. Once it's consumed — typically 4–6 years in — the tank itself starts corroding. Replacing it mid-life is the single best tank-life extender.
Water-heater terms connect to tank vs tankless decisions, recovery rate for busy households, anode-rod maintenance against our region's water chemistry, and code requirements like expansion tanks.