A balk is a term used in baseball when a pitcher performs an illegal action on the pitcher's mound. This usually is a fake pitch.
If a pitcher attempts to deceive a batter with a false pitch, drops the ball or makes any other unauthorized movement while engaged on the rubber, a balk may be called. If a pitcher begins a throwing motion, he must follow through with the action, or the umpire may deem it a violation.
When a balk is committed, it results in a dead ball, meaning the current play is declared over, and no runners may be tagged or thrown out. Any runners on base when the balk is called are entitled to move up a base.