MashGet

Backlash from WikiPedia:

In mechanical engineering, backlash, sometimes called lash or play, is clearance between mating components, sometimes described as the amount of lost motion due to clearance or slackness when movement is reversed and contact is re-established. For example, in a pair of gears, backlash is the amount of clearance between mated gear teeth. In other words it is the difference between the tooth space and the tooth thickness, as measured along the pitch circle. Theoretically, the backlash should be zero, but in actual practice some backlash must be allowed to prevent jamming of the teeth due to tooth errors and thermal expansion. This gap means that when a gear-train is reversed the driving gear must be turned a short distance before all the driven gears start to rotate. A similar effect is the taking up of slack when a train starts to pull away at the station and each car bumps as its link becomes tight. At low power outputs, backlash results in inaccurate calculation from the small errors introduced at each change of direction; at large power outputs backlash sends shocks through the whole system and can damage teeth and other components.

Read more on WikiPedia