A body of water interposed as a bar to the passage or escape of gas.(noun)
The Century Dictionary (Public Domain)
Use "water-seal" in a sentence
"The tunnel must dip then come up like the water-seal trap under a kitchen sink."
"For example, water-seal slabs in latrines reduce the breeding sites for culicine mosquitos, vectors of filariasis; treatment of excrete prior to its disposal can kill the eggs and cysts of many human parasites (Ascaris, Entamoeba, and Schistosoma spp), thus preventing contamination of both ground and water."
"Fixed-dome plants are regarded as leaktight if the water-seal test shows less than 2% water loss, and the gas-seal test shows less than 5% gas loss."