The definition of a cistern is a tank for storing water, or is an underground reservoir for rainwater.(noun)
The toilet tank that holds the water used to flush the toilet is an example of a cistern.
See cistern in Webster's New World College Dictionary
Origin: ME cisterne > OFr < L cisterna, reservoir for water < cista, chest
See cistern in American Heritage Dictionary 4
Origin: Middle English cisterne
Origin: , from Latin cisterna
Origin: , from cista, box
Origin: , from Greek kistē, basket.
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