To
paraphrase Webster, backlash is a
nonlinear phenomenon you find in a
loose gearbox. Try turning the
drive shaft clockwise. Then,
stop, and turn it slightly
counterclockwise. if you encounter a small region of no
output in which the
gears have to turn a little bit to have the teeth touch, then you are experiencing
backlash.
Backlash is why some precision equipment is so annoying to use. In an electronics lab, I had an apparatus that would measure resistance or luminosity or something like that in a black box, and I had to turn a little crank to make the sensor mount move a specific distance that was measured by an encoder wheel on the crank. I had to take a measurement at specific intervals. If I overshot one part, I had to start all over again because if I backtracked, the resulting backlash would introduce error into my readings.
Backlash should not be confused with a deadband, which basically erases all signal below a specific theshold. Backlash is different because it only occurs when you reverse the input signal.