Definition
A magnet suspended in liquid inside a magnetic compass so it can rotate freely and align itself with the Earth's magnetic field. In a vertical card magnetic compass, the floating magnet is attached to a vertical compass card, which the pilot reads through a window on the instrument face.
Plain English
A small magnet that floats in fluid inside the compass. Because it floats freely, it can swing around and point toward magnetic north, turning the compass card so the pilot can read the heading.
Context Anchor
Seen in descriptions of the vertical card magnetic compass and how its internal parts turn to show heading.
Derivation
"Floating" describes the magnet's suspension in liquid (typically a kerosene-like fluid), which lets it rotate with very little friction. The fluid also damps out sudden swings so the magnet settles quickly on a heading rather than spinning back and forth.
Why Pilots Care
Reduces friction and oscillation so the compass gives stable, readable headings during turns and turbulence.
Intuition Check
Do not picture a loose magnet drifting around inside the compass. In this context, floating means supported and free to turn, while still kept in the proper position inside the instrument.
Example Sentence 1
During the preflight check, the pilot tapped the compass housing and watched the floating magnet settle smoothly on the runway heading.
Example Sentence 2
Before takeoff the pilot checked that the floating magnet moved freely when the aircraft was turned through north.