Definition
A permanent magnet bent into the shape of a U or horseshoe so that its two opposite poles, north and south, sit close together at the open end. This shape concentrates the magnetic field in the gap between the poles, producing a strong, usable field in a small space.
Plain English
A bar magnet bent so its two ends face each other. The magnetic pull is strongest in the gap between those ends, which is where the useful work gets done.
Context Anchor
Seen in basic aircraft electrical and ignition-system discussions, especially when explaining how magnets create a magnetic field.
Derivation
Named simply for its shape -- it looks like a horseshoe. The U-bend is not decorative; it brings the two poles close together so the magnetic field lines have a short path to cross, which makes the field much stronger in that gap than it would be along a straight bar magnet.
Why Pilots Care
The magneto that fires your spark plugs uses permanent magnets shaped to concentrate a strong field in a small gap. Understanding why the shape matters helps you understand how a magneto can generate ignition voltage with no battery and no outside electrical power -- which is why the engine keeps running if the electrical system fails.
Intuition Check
A horseshoe magnet is not connected with horses in its function. The word “horseshoe” only describes the U shape of the magnet.
Example Sentence 1
Early magnetos used a horseshoe magnet to produce the strong field needed to induce ignition voltage in the coil.
Example Sentence 2
The horseshoe magnet in the magneto rotates past the coil to induce the voltage needed for engine starting.