Plain English Meaning
A small knob or switch in the cockpit used to nudge the heading display so it lines up correctly with magnetic north.
Full Definition
A cockpit control on a remote indicating compass system that allows the pilot to manually adjust the compass card to match magnetic north when the system is operated in the free gyro (non-slaved) mode, or to correct the heading indicator if it drifts out of agreement with the flux valve.
Context Anchor
Seen in instrument flying when using a remote indicating compass or heading system that can operate in slaved or free mode.