Definition
In a vertical card magnetic compass, a magnet attached directly to the shaft that drives the compass card, so that as the magnet aligns itself with Earth's magnetic field, it rotates the shaft and the card displays the aircraft's heading.
Plain English
A small magnet fixed to a turning rod inside the compass. The magnet swings to point north, and because it is fixed to the rod, the rod turns the heading card you read.
Context Anchor
Seen in descriptions of how a vertical card magnetic compass moves its display.
Derivation
Shaft-mounted' simply means attached to a shaft -- a rotating rod. The point of the term is that the magnet and the shaft turn as one unit, so the magnet's movement directly drives the compass card.
Why Pilots Care
It supplies the compass with its magnetic reference so the pilot receives accurate heading information without friction or lag from the card assembly.
Analogy
Think of a small bar magnet fixed to a thin turning rod. When the magnet swings to line up with north and south, the rod turns whatever is attached to it.
Intuition Check
Do not read “shaft-mounted” as meaning an engine shaft. In this compass context, the shaft is a small turning rod inside the compass instrument.
Example Sentence 1
The vertical card magnetic compass uses a shaft-mounted magnet to rotate the heading card as the aircraft turns.
Example Sentence 2
During a compass check, the mechanic verified that the shaft-mounted magnet rotated freely without binding.