Plain English Meaning
A way of measuring air pressure by comparing it to how high it would push a column of liquid mercury in a tube. The higher the pressure, the higher the mercury rises, measured in inches.
Full Definition
A unit of pressure measurement equal to the pressure exerted by a column of mercury one inch tall at a standard temperature and gravity. In aviation, it is the standard unit for altimeter settings and manifold pressure in piston engines. Standard sea-level atmospheric pressure is 29.92 inches of mercury.
Context Anchor
You may see inches of mercury during a before-takeoff check on the engine pressure gauge, and in weather or altimeter settings such as 29.92 inches of mercury.