A tank that holds the water used to flush a toilet(noun)
WordNet. Princeton University. 2010.
Use "water tank" in a sentence
""Are you aiming to put in a water tank at Plum Creek?""
""Just where the railroad's going to cross -- I reckon maybe they're aiming to put in a water tank there.""
"Just across the garden from the summer kitchen is the sauna, another half-cob, half-wooden shed, heated by a wood-stove with a water tank perched above it."