The amount of inaccuracy that is introduced to a cut when a tool reverses direction. Because the fit of each component of a drive system is not ever going to be perfect, there will be a brief period while a motor reverses direction where the components of the drive system are spinning freely until they all engage in the reverse direction. Because there is no way to reliably predict the amount of backlash that each movement causes--CNC controllers can only assume that backlash does not occur and accept any inaccuracy caused by this phenomenon.
Backlash is typically expressed in units of distance. This is the distance that the tool should have moved in the reverse direction during the period where the motor was spinning freely waiting to engage in the reverse direction.
Typically, lead screw drives experience less backlash than rack and pinion drives.