Plain English Meaning
Pieces of metal in the aircraft that have become slightly magnetic themselves and pull the compass needle off true magnetic north.
Full Definition
Metal components within the aircraft that have acquired their own magnetic field, causing them to influence the magnetic compass and produce a compass error known as deviation. The magnetism may come from ferrous metals near the compass, electrical currents flowing through wiring, or steel parts that have become magnetized over time through exposure to other magnetic fields or vibration.