A balk is an illegal pitch in baseball that can be called by any umpire at any level of the game. It is most commonly called when a pitcher makes a motion to deliver a pitch while either one or more runners are on base, or when the pitcher attempts an illegal quick pitch. The balk essentially keeps the pitcher in check by limiting his ability to fake a pitch or a pick-off throw to one of the bases. If an umpire deems the pitcher pretended to do either, without clear intention of following through, the balk is called and each of the runners move up one base. Some examples of illegal motions or actions that constitute a balk include:

• Making any sort of motion that mimics the pitchers normal delivery, and then stopping before firing the pitch.
• Throwing a pitch right after getting the ball back from the catcher, or while at least one of the catchers feet is out of the box.
• Faking a pickoff throw to one base before firing to another.
• Making a motion to deliver a pitch and then not following through.

If a balk is enforced, the pitch is generally nullified, each runner is awarded one base, and the batter remains at bat with the previous count. The balk rule in Major League Baseball was introduced in 1898 to prevent the pitcher from deliberately deceiving the base runner. The specific rules for balks were first introduced to prevent pitchers from intentionally deceiving baserunners.