A wheel with teeth or projections around its edge that fit into another toothed wheel to transfer rotational motion in machinery.
From 'cog' (a tooth on a wheel) combined with 'wheel,' a straightforward English compound describing a mechanical device.
The cogwheel is one of humanity's fundamental inventions—nearly every machine from watermills to smartphones relies on this principle, and the word itself is over 600 years old, unchanged because the device is so essential.
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