Cistern

Cistern: A tank used to collect rainwater runoff from the roof of a house or building
Clarity: The clearness of a liquid.

Cistern
Small tank or storage facility used to store water for a home or farm; often used to store rain water.
Clarification ...

cistern A small tank, usually covered, or a storage facility used to store water for a home or farm. Often used to store rain water.

Grey Water is wastewater from household use, such as sink and bathrooms, that is being re-used, e.g as cistern flush water. Black water is effluent from household use being used for cistern flushing.

RESERVOIR: A body of water used to collect and store water, or a tank or cistern used to store potable water.