Cistern

Noun

 * 1) A reservoir or tank for holding water, especially for catching and holding rainwater for later use.
 * 2)  In a flush toilet, the container in which the water used for flushing is held; a toilet tank.
 * 3)  A cisterna.

Thesaurus
artificial lake, bayou lake, dam, dead water, dike, etang, farm pond, fishpond, freshwater lake, glacial lake, inland sea, lagoon, laguna, lake, lakelet, landlocked water, linn, loch, lough, mere, millpond, millpool, nyanza, oxbow lake, plash, pond, pondlet, pool, puddle, reservoir, salina, salt pond, stagnant water, standing water, still water, sump, tank, tarn, tidal pond, volcanic lake, water hole, water pocket, well