Plain English Meaning
It is the reading on the manifold pressure gauge, measured in inches of mercury, that shows how hard the engine is being worked by the throttle.
Full Definition
A unit used to measure manifold pressure in a piston aircraft engine, expressed in inches of mercury (in. Hg). It indicates the absolute pressure of the air-fuel mixture inside the engine's intake manifold, as shown on the manifold pressure gauge in aircraft equipped with a constant-speed propeller.
Context Anchor
Seen during power setting changes in airplanes with constant-speed propellers, such as takeoff, climb, cruise, descent, and engine management .