A mechanical device with a pawl (hinged catch) that allows movement in one direction only, stopping backward motion.
From French 'rochet' (bobbin, spool), possibly from Old Norse or Germanic sources. The mechanical tool term appeared in English around 1600s for devices that prevent reverse motion.
Ratchet straps and buckles use the ratchet principle to let you tighten indefinitely but not loosen accidentally—the same mechanism was used in medieval siege weapons!
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