Definition
The small amount of free play between the meshed teeth of two gears, measured as the distance one gear can move while the other is held stationary. A specified amount of backlash is required to allow for thermal expansion, lubrication, and manufacturing tolerances; too little causes binding, while too much causes wear, noise, and inaccurate motion transfer.
Plain English
The tiny bit of slack between two gears where one can wiggle slightly before it pushes the other. A small amount is needed so the gears don't bind, but too much means sloppy, worn-out motion.
Context Anchor
Seen in aircraft maintenance when checking engine gears, accessory drives, landing gear mechanisms, or other gear-driven parts.
Derivation
Backlash' combines 'back' (reverse direction) with 'lash' (a sudden movement or strike). The term originally described the sharp reverse motion felt when a driving gear reversed direction and took up the slack before re-engaging the driven gear.
Why Pilots Care
Excessive backlash accelerates wear or causes failure in reduction gears and accessory drives.
Analogy
It is like a loose door handle: you can turn it slightly before the latch actually starts to move. That small free movement is similar to backlash between gears.
Intuition Check
Backlash does not mean a negative reaction here. Here it means mechanical looseness or clearance between gear teeth.
Example Sentence 1
The mechanic measured gear backlash in the magneto drive and found it within the manufacturer's tolerance.
Example Sentence 2
Proper gear backlash prevents binding in the constant-speed propeller drive during high-power climbs.