What is the gear backlash?

The backlash of the gear is when a pair of gears mesh, the gap between the tooth surfaces.
Proper backlash design is one of the key factors to smooth gear operation.

The general purpose of backlash is to prevent gears from jamming and making contact on both sides of their teeth simultaneously. Any error in machining which tends to increase the possibility of jamming makes it necessary to increase the amount of backlash. Consequently, the smaller the amount of backlash, the more accurate must be the machining of the gears. Runout of both gears, errors in profile, pitch, tooth thickness, helix angle and center distance - all are factors to consider in the specification of the amount of backlash. In order to obtain the amount of backlash desired, it is necessary to change the tooth thickness or tooth space allowances