a hinged catch that fits into a notch of a ratchet to move a wheel forward or prevent it from moving backward
source: WordNet 3.0
A pivoted tongue, or sliding bolt, on one part of a machine, adapted to fall into notches, or interdental spaces, on another part, as a ratchet wheel, in such a manner as to permit motion in one direction and prevent it in the reverse, as in a windlass; a catch, click, or detent. See Illust. of Ratchet Wheel.