Wiktionary
n. (context baseball English) The moment when the pitcher begins the motion of pitching the ball and, by the rules of the game, has committed himself to throwing the pitch.
Wikipedia
In baseball, the time of pitch is that instant when the pitcher has begun his pitching motion and, by the rules, has committed himself to throwing the pitch. This instant thus occurs before the pitcher actually releases the ball. Once a pitcher commits himself to throwing a pitch, it is illegal for him to return to his set. If that happens, he is charged with a balk, and any baserunners are allowed to advance a base.
A baserunner's time-of-pitch base is the base which he has last legally reached at the time of pitch.
Category:Baseball rules