Definition
A drain plug fitted into the bottom of an engine, gearbox, or transmission sump that contains a magnet. The magnet attracts and holds ferrous (iron-based) particles suspended in the oil so they can be inspected when the plug is removed during servicing. The presence, quantity, and character of metal collected on the plug gives an early indication of internal wear or impending component failure.
Plain English
A drain plug with a built-in magnet that catches tiny iron particles floating in the engine oil. When mechanics pull the plug, they look at what stuck to it -- a small amount of fuzz is normal, but chunks or flakes mean something inside is wearing or breaking.
Context Anchor
Seen during aircraft engine oil changes, maintenance inspections, and discussions of engine wear.
Derivation
Magnetic comes from magnet, originally connected with lodestone, a naturally magnetic rock known from the region of Magnesia. Drain plug describes a removable plug at a low point where fluid can be drained. Together, the words point to a drain plug that also acts like a small metal catcher.
Why Pilots Care
Early detection of internal wear allows maintenance action before serious engine damage occurs.
Analogy
It works like a small magnet placed in a stream of oil. As oil moves past it, iron or steel particles stick to the magnet instead of continuing to circulate.
Intuition Check
Do not think of it as a special electrical part or sensor. It is a normal drain plug with a magnet added to catch metal particles.
Example Sentence 1
During the oil change, the mechanic removed the magnetic drain plug and found only a light fuzz of normal wear particles.
Example Sentence 2
Before the next flight the pilot checked the magnetic drain plug for any unusual ferrous debris.