noun

• (literal) A suddenly reversed or backward motion, such as of a rope or elastic band when it snaps under tension.
• (figurative) A negative reaction, objection or outcry, especially of a violent or abrupt nature.
  "The public backlash to the proposal was quick and insistent."
• (mechanics) The looseness through which one part of connected machinery, such as a wheel, gear, piston, or screw, can be moved without moving the connected parts, or a measurement of the distance moved thereby; either intentional (as allowance) or unintentional (from error or wear).
  "... the word "backlash" is used in the extended sense in which it is understood in Lancashire and Yorkshire , including not only backlash proper , but also break of contact between the working faces of the teeth of the two wheels when the driven wheel overruns the driver without actual backlash . To place rope or belt drums of any considerable size upon shafts driven by spur gearing is nearly always dangerous."
• (mechanics) The jarring or reflex motion caused in badly fitting machinery by irregularities in velocity or a reverse of motion.
  "In ...breakages from high speed and irregular pitch, ten out of the eleven wheels had spur segments moulded from patterns, instead of by machinery; and in three of the ten the teeth were cast upon the felks, and were twisted and drawn, as they generally are when cast in this way, by the contraction of the mass of metal of which they form a part. In every instance the speed was high, and in several cases there was more or less backlash or over-running."