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Ratchet
A mechanism which will rotate freely ("freewheel") in one direction, but either will not turn the other way, or will drive some other part when turned the other way.
A bicycle's freewheel has a ratchet which causes the sprocket to turn the wheel forward when the rider pedals forward, but allowing the wheel to turn forward even when the pedals are not being turned.
Ratchet wrenches turn a nut or bolt only when moved in one direction, leaving the nut or bolt in place when turned the other way, allowing the wrench to be used by swinging it back and forth, without having to lift it off the nut or bolt for the backswing.
A ratchet was also part of the Sun Tour Power Ratchet shift levers. Most ratchets use spring-loaded pawls to engaged a toothed ring for forward drive, while the teeth in the ring slide over the ends of the pawls when freewheeling.
