cistern
n. countablen. a large tank used to store water. You often see these on the roofs of buildings or in gardens to collect rainwater.
n. a large tank or reservoir for storing water, typically located on a roof or in a basement. Often used for collecting rainwater or providing a secondary water supply.
The garden fountain is fed by a large cistern on the roof.
During the long summer drought, the residents relied on the rooftop cistern to provide water for their vegetable patch.
The ancient city's sophisticated water management system included a series of underground cisterns designed to collect and filter rainwater for the entire population.
From Middle English cisterne, from Old French cisterne (Modern French citerne) from Latin cisterna, from cista (“box”), from Ancient Greek κίστη (kístē, “box”). Doublet of cisterna.