cistern cistern /ˈsɪstɝn/

noun

1. An artificial reservoir for storing liquids; especially an underground tank for storing rainwater  ( cistern )
   an artificial reservoir for storing liquids; especially an underground tank for storing rainwater
    They built a cistern to collect rainwater and use it for household chores during the dry season.
   They built a cistern to collect rainwater and use it for household chores during the dry season.
   
   
2. A tank that holds the water used to flush a toilet  ( cistern , water tank )
   a tank that holds the water used to flush a toilet
   
3. A sac or cavity containing fluid especially lymph or cerebrospinal fluid  ( cistern , cisterna )
   a sac or cavity containing fluid especially lymph or cerebrospinal fluid