Backlash \Back"lash`\, n. [Back, adv. + lash.]

(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.

Source: Webster's Revised Unabridged Dictionary (1913)