To remove or reduce the magnetic properties of something.
From 'de-' (remove) + 'magnetise' (British spelling, to make magnetic). 'Magnetise' comes from 'magnet.' This technical verb entered English in the 1800s with electrical and magnetic science.
British scientists pioneered magnetism research—they called it 'magnetising' and 'demagnetising,' and those '-s' spellings stuck in physics and engineering labs worldwide, even in American institutions.
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