Definition
The small amount of free movement, or lost motion, between two mating mechanical parts (such as gear teeth or a control linkage) when the direction of motion is reversed. It is the slack that must be taken up before the driven part responds to a change in direction of the driving part.
Plain English
The little bit of slop or play you feel when you reverse the direction of two parts that work together, like gears or a control rod. Push one way, then the other, and there's a small gap before the second part starts moving.
Context Anchor
Seen in aircraft maintenance, flight control rigging, engine accessory drives, and other mechanical systems where linked parts must move together accurately.
Derivation
From 'back' (reverse direction) plus 'lash' (a sudden movement or stroke). The word originally described the snap or jerk felt when motion reverses through a loose mechanical connection. That sense carries directly into the aviation meaning: the small lurch or play before parts re-engage in the opposite direction.
Why Pilots Care
Excessive backlash in control systems produces imprecise handling, delayed response, or potential flutter that affects aircraft safety and handling qualities.
Analogy
It is like turning a loose doorknob and feeling it wiggle slightly before the latch actually starts to move.
Intuition Check
Backlash does not mean a negative reaction here. In aircraft mechanical use, it means looseness or free movement between connected parts.
Example Sentence 1
During the inspection, the mechanic checked the elevator control system for excessive backlash before signing off the aircraft.
Example Sentence 2
Too much backlash in the propeller governor linkage caused unstable engine RPM during cruise.