Definition
The tendency of a magnetic material to retain some of its magnetism after the magnetizing force has been removed, causing the material's magnetic state to lag behind changes in the applied magnetic field.
Plain English
When you magnetize a piece of iron and then remove the magnet, the iron stays a little bit magnetic. It doesn't instantly snap back to zero. That lag between cause and effect is magnetic hysteresis.
Context Anchor
Seen in discussions of aircraft magnetic compasses, compass error, and magnetized metal parts in or near the aircraft.
Derivation
From the Greek hysteresis, meaning 'a coming late' or 'lagging behind.' The name fits: the material's magnetism lags behind the magnetizing force that's acting on it.
Why Pilots Care
Hysteresis is the reason a magneto can keep producing usable spark even as the magnetic field collapses, and it's also why magnetic compasses and other iron components can pick up small residual magnetism that throws off readings until they're swung or demagnetized.
Analogy
Think of memory foam. Press your hand into it and lift away — the foam doesn't instantly return to flat. The shape lingers. Magnetic hysteresis is the same idea, but with magnetism instead of shape.
Grounding Statement
Magnetic hysteresis is the magnetic “memory” of certain metal parts after a magnetic force has changed or been removed.
Intuition Check
Do not read hysteresis as simply “magnetism.” In this term, it means magnetism that lags behind or remains after the outside magnetic force changes.
Example Sentence 1
Residual magnetism in the engine's iron components is caused by magnetic hysteresis, which is why the magneto can produce a spark even before the field fully builds.
Example Sentence 2
Materials with low magnetic hysteresis are chosen for aircraft instruments to reduce unwanted energy loss and heat.