water is kept in a cistern

A cistern can be a wide range of shapes & sizes. Most commonly a cistern is round or rectangular.

The Arabic word for cistern is صهريج.

Underground water chambers: Cisterns from Latin cisterna

http://en.wikipedia.org/wiki/Cistern

an example is the Basilica Cistern under Istanbul

Also reservoirs were used for large scale storage as basically a man-made lake, water would then be transported via aqueducts to where it was needed.

http://en.wikipedia.org/wiki/Reservoir

http://en.wikipedia.org/wiki/Roman_aqueduct

A tank that overflows. Cistern is an English name for tank.