Very simply, this is some receptacle that holds water. Generally speaking, they are immovable – a bucket could be a cistern, but that’s not how the word is used. A cistern is usually considered to a be a built-in feature of a building.
A cistern is filled, either by water delivery or by rainwater gathering. It might be raised from the building – or on its roof – in order to provide water pressure. A cistern is not open, because then the water would be easily contaminated and would evaporate.
Cistern water is not safe to consume directly. It would need to be filtered or boiled first.
Why I Looked It Up
I was in the Dominican Republic, staying in an apartment building in Navarette. The building was not connected to the city water supply (I’m not sure that the city even had a water supply).
Instead, it had a cistern under the building. Periodically, you could hear a water pump that would move water to a smaller tank on the top of the building. Gravity from this roof tank would supply water pressure to the entire building – all the faucets, toilets, and showers ran from water pushing down out of this tank that was constantly being refilled from below.
(Why not just have all the water on the roof to start with? I assume it would be too heavy, but I’m not sure.)
We were told there was limited water supply. When the cistern was empty, there was no more water until it could be refilled. (Presumably, water that flushed or went down a sink emptied into the city sewer system, so it was one-way.)
On the last day there, a water truck showed up to refill the cistern. It had a big tank, and it literally shoved a big hose into a hole in the first floor, and pumped water into it. I don’t know how often this had to happen.
I don’t believe there was any rainwater collection in this particular building. The cistern had to be manually and periodically refilled.
We were warned not to drink from any tap because the cistern water was not pure. Drinking water was delivered in five gallon bottles.