Balk

A ruling made by an umpire against a pitching motion that violates rules intended to prevent the pitcher from unfairly deceiving a baserunner. When a balk is called, each runner can freely advance one base. There are specific guidelines in the rules about what pitching movements are illegal. The spirit of a balk is that certain movements effectively "declare" the pitcher's intent to pitch to the batter, freeing the baserunner from any fear that he will be picked off. Some balks result from errant or unsuccessful motions, in the case of the ball accidentally slipping out of the pitcher's hand; this usage is likely linked to its use outside of baseball, to mean any process that has been prematurely blocked.