Plain English Meaning
A way of measuring air pressure. The number tells you how high a column of mercury that pressure can push up, measured in inches.
Full Definition
A unit of pressure measurement equal to the pressure exerted by a column of mercury one inch high at standard temperature and gravity. In aviation, it is the standard unit used to express atmospheric pressure for altimeter settings and manifold pressure in piston engines.
Context Anchor
Seen on altimeter settings, weather reports, and manifold pressure gauges during engine checks and before-takeoff checks.
Derivation
The double-quote symbol (") means inches. Hg is the chemical symbol for mercury, taken from the Latin hydrargyrum, meaning 'liquid .' Together, "Hg literally means '' — a unit that dates to early barometers, which measured air pressure by how high it pushed a column of mercury up a tube.