Time of pitch
In baseball, the time of pitch is the 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 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 all baserunners advance a base.
A baserunner's time-of-pitch base is the base which he has last legally reached at the time of pitch.