A balk is an illegal motion that a pitcher makes that are designed to deceive base runners - making them they believe the pitcher will pitch when he has no intention of doing so (this is overly simplistic, but the full explanation is below).
If a pitcher balks, base runners are awarded one base.
The MLB record for most balks in a season is 16 by Dave Stewart of the Oakland A’s in 1988. The season record is 90 by Steve Carlton. And Bob Shaw of the Milwaukee Brewers had five in one game against the Cubs on May 4, 1963 - a MLB single-game record.
Here’s what a book looks like:
Finally, here’s the balk section of the MLB rule book:
If there is a runner, or runners, it is a balk when —
(a) The pitcher, while touching his plate, makes any motion naturally associated with his pitch and fails to make such delivery;
Rule 8.05(a) Comment: If a lefthanded or righthanded pitcher swings his free foot past the back edge of the pitchers rubber, he is required to pitch to the batter except to throw to second base on a pick-off-play.
(b) The pitcher, while touching his plate, feints a throw to first base and fails to complete the throw;
(c) The pitcher, while touching his plate, fails to step directly toward a base before throwing to that base;
Rule 8.05(c) Comment: Requires the pitcher, while touching his plate, to step directly toward a base before throwing to that base. If a pitcher turns or spins off of his free foot without actually stepping or if he turns his body and throws before stepping, it is a balk.
A pitcher is to step directly toward a base before throwing to that base but does not require him to throw (except to first base only) because he steps. It is possible, with runners on first and third, for the pitcher to step toward third and not throw, merely to bluff the runner back to third; then seeing the runner on first start for second, turn and step toward and throw to first base. This is legal. However, if, with runners on first and third, the pitcher, while in contact with the rubber, steps toward third and then immediately and in practically the same motion wheels and throws to first base, it is obviously an attempt to deceive the runner at first base, and in such a move it is practically impossible to step directly toward first base before the throw to first base, and such a move shall be called a balk. Of course, if the pitcher steps off the rubber and then makes such a move, it is not a balk.
(d) The pitcher, while touching his plate, throws, or feints a throw to an unoccupied base, except for the purpose of making a play;
(e) The pitcher makes an illegal pitch;
Rule 8.05(e) Comment: A quick pitch is an illegal pitch. Umpires will judge a quick pitch as one delivered before the batter is reasonably set in the batters box. With runners on base the penalty is a balk; with no runners on base, it is a ball. The quick pitch is dangerous and should not be permitted.
(f) The pitcher delivers the ball to the batter while he is not facing the batter;
(g) The pitcher makes any motion naturally associated with his pitch while he is not touching the pitchers plate;
(h) The pitcher unnecessarily delays the game;
Rule 8.05(h) Comment: Rule 8.05(h) shall not apply when a warning is given pursuant to Rule 8.02(c) (which prohibits intentional delay of a game by throwing to fielders not in an attempt to put a runner out). If a pitcher is ejected pursuant to Rule 8.02(c) for continuing to delay the game, the penalty in Rule 8.05(h) shall also apply. Rule 8.04 (which sets a time limit for a pitcher to deliver the ball when the bases are unoccupied) applies only when there are no runners on base.
(i) The pitcher, without having the ball, stands on or astride the pitchers plate or while off the plate, he feints a pitch;
(j) The pitcher, after coming to a legal pitching position, removes one hand from the ball other than in an actual pitch, or in throwing to a base;
(k) The pitcher, while touching his plate, accidentally or intentionally drops the ball;
(l) The pitcher, while giving an intentional base on balls, pitches when the catcher is not in the catchers box;
(m)The pitcher delivers the pitch from Set Position without coming to a stop.
PENALTY: The ball is dead, and each runner shall advance one base without liability to be put out, unless the batter reaches first on a hit, an error, a base on balls, a hit batter, or otherwise, and all other runners advance at least one base, in which case the play proceeds without reference to the balk.
APPROVED RULING: In cases where a pitcher balks and throws wild, either to a base or to home plate, a runner or runners may advance beyond the base to which he is entitled at his own risk.
APPROVED RULING: A runner who misses the first base to which he is advancing and who is called out on appeal shall be considered as having advanced one base for the purpose of this rule. Rule 8.05 Comment: Umpires should bear in mind that the purpose of the balk rule is to prevent the pitcher from deliberately deceiving the base runner. If there is doubt in the umpires mind, the intent of the pitcher should govern. However, certain specifics should be borne in mind:
(a) Straddling the pitchers rubber without the ball is to be interpreted as intent to deceive and ruled a balk.
(b) With a runner on first base the pitcher may make a complete turn, without hesitating toward first, and throw to second. This is not to be interpreted as throwing to an unoccupied base.