n. [ Back, adv. + lash. ] 1. (Mech.) The distance through which one part of connected machinery, as a wheel, piston, or screw, can be moved without moving the connected parts, resulting from looseness in fitting or from wear; also, the jarring or reflex motion caused in badly fitting machinery by irregularities in velocity or a reverse of motion. [ 1913 Webster ]

2. A strong and sudden reverse movement in a moving part of a machine. [ PJC ]

3. A strong popular reaction serving to counter the effect of an action; -- used especially of adverse reactions to social or political developments. [ PJC ]