a container in which water is stored in a building, especially one in the roof or connected to a toilet （尤指屋顶上的）蓄水箱，贮水箱；（抽水马桶的）水箱 
Oxford Collocations DictionaryCistern is used after these nouns:- lavatory
Word OriginMiddle English: from Old French cisterne, from Latin cisterna, from cista ‘box’, from Greek kistē.
