Plain English Meaning
A pressure-measuring device that uses a tube of mercury standing in a small pool. The heavier the air outside, the higher it pushes the mercury up the tube, and you read the pressure off the height of that column.
Full Definition
An instrument that measures atmospheric pressure by the height of a column of mercury supported in a vertical glass tube whose lower end sits in an open reservoir of mercury. The weight of the surrounding air pressing down on the reservoir pushes mercury up the tube, and the height of the column — typically expressed in inches or millimeters of mercury — indicates the current atmospheric pressure. Standard sea-level pressure on this instrument reads 29.92 inches of mercury (1013.2 millibars).
Context Anchor
Seen in discussions of atmospheric pressure, standard pressure, and how pressure information is used in aviation weather and settings.