Cistern/ˈsɪstərn/ Definition 1 of 3Pro Tip 1/3Common Usage Mostly used to describe the tank in a flush toilet or for rainwater collection. The cistern in our backyard collects rainwater. Pro Tip 2/3Fixed Location Cisterns are typically stationary and part of a building's plumbing system. The cistern is installed in the attic. Pro Tip 3/3Capacity Implication A cistern implies a larger capacity than smaller household water containers. They replaced the small tank with a cistern to avoid water shortages.