The reverse flow of water or other liquid in a pipe or system, opposite to its normal direction.
From 'back' + 'flow,' a technical term that became standard in plumbing and water treatment in the 20th century.
Backflow in water systems can contaminate entire municipal water supplies—if sewer water flows backward into drinking water due to pressure changes, it can sicken thousands, which is why building codes now require expensive prevention devices.
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