cistern

US /ˈsɪs.tɚn/
UK /ˈsɪs.tɚn/
"cistern" picture
1.

an underground reservoir for storing rainwater

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The old house had a large cistern for collecting rainwater.
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They relied on the cistern during the dry season.
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reservoirtankwellbasin
2.

a tank holding water for flushing a toilet

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The plumber had to fix the leaky cistern in the bathroom.
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You can hear the water refilling the cistern after flushing.
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flush tanktoilet tank