Backlash is a form of slack or play (a dead space when reversing direction of motion) found in almost any mechanical system. For example, backlash in gears is caused by the tolerance or gaps between the teeth. When the mechanism reverses direction that slack needs to be taken up before the next gear in the sequence engages. Although excessive backlash is bad a little backlash is required to prevent jamming and to allow the gears to reverse direction. With backlash in a gear train a full turn of a gear forwards and then backwards may not leave the other gears in the same location.
Because backlash causes a dead zone it can affect focus or rotation. Backlash that is less then the critical focus zone (the area where a change in position does not change the focus) has no affect on focus, however fast systems may need to correct for it. This can be accomplished with backlash compensation.
Backlash Compensation works by ensuring that the device always ends a move coming from the same direction. That way, regardless of how the device reached a specific position, the gears are engaged in the same way. This takes up any slack in the mechanism. This can be accomplished by overshooting the target and then coming back for moves in the opposite of the approach direction.