cis·tern 
/ˈsɪstɚn/ noun
plural cis·terns /ˈsɪstɚn/ noun [count] : a container that holds a supply of water: such as a US : an underground container that is used for collecting and storing rainwater
b Brit : a tank that holds the water for a toilet
c Brit : a tank on the roof of a building that holds water for the building
b Brit : a tank that holds the water for a toilet
c Brit : a tank on the roof of a building that holds water for the building
