Definition
A sealed reservoir of mercury that forms the lower reference end of a mercury barometer. Atmospheric pressure pushes down on the mercury in the cistern, forcing it up a closed glass tube; the height of the mercury column above the cistern surface is read as the current barometric pressure.
Plain English
The small pool of mercury at the bottom of a mercury barometer. The air pressing on this pool is what pushes the mercury up the tube, and how high it rises tells you the air pressure.
Context Anchor
Seen in aviation weather and instrument theory when mercury barometers are described.
Derivation
From the Latin cisterna, meaning a tank or container for holding liquid. The aviation use keeps that original sense -- it is simply the small holding tank of mercury at the base of the instrument.
Why Pilots Care
Pilots usually do not handle mercury barometers in the cockpit, but the term can appear when learning how atmospheric pressure is measured and how pressure information supports altimeter settings.
Intuition Check
Do not read cistern here as a building’s water tank. In this context, it means the liquid-holding part of a pressure-measuring instrument, usually a mercury barometer.
Example Sentence 1
As atmospheric pressure rises, the air pushes harder on the mercury in the cistern, driving the column higher up the tube.
Example Sentence 2
Water from the cistern supplies the aircraft lavatory system.