What is a balk in baseball?

A balk is any of a large number of illegal actions made by a pitcher when he is on the rubber. When this happens, the ball is declared dead and all base runners are awarded the next base. The main reason for the rule is give a runner a fair change to steal a base. Simply put, a balk is a fake pitching move- an attempt to make it look to the runner like a pitch in being made when in fact the ball is being held in the pitcher's hand or going to a base.

It is also a balk when the catcher is out of the catcher's box when the pitcher begins his pitch, and when any player other than the catcher is not in fair territory when the pitcher begins his pitch.