Plain English Meaning
A way of measuring pressure by comparing it to how high it would push a column of mercury in a tube. The taller the column, the higher the pressure. Aviation uses this unit on altimeters and on the manifold pressure gauge in airplanes with constant-speed propellers.
Full Definition
A unit of pressure measurement equal to the pressure exerted by a column of mercury one inch tall at standard temperature and gravity. In aviation it is the standard unit for reporting manifold pressure, altimeter settings, and atmospheric pressure. Standard sea-level atmospheric pressure is 29.92 "Hg.
Context Anchor
Seen when discussing manifold pressure, engine power settings, and constant-speed propeller operation.