| Baseball jargon. Refers to a player's ability to draw a walk, and to avoid swinging at pitches outside the strike zone.
The seminal skill of baseball offense is the ability to reach base. Hits and walks are both valid methods for doing this. Plate discipline dramatically affects both of these, as follows:
- Hitters' batting average, on-base percentage, and slugging percentage are consistently higher when they are ahead in the count, and lower when they're behind in the count. This effect has been documented for years, and is significant even when comparing hitters' performance following a 0-1 count to their performance following a 1-0 count. This metric also applies to pitchers, so it is reasonable to conclude that the results for hitters are not solely due to an uneven division of the quality of pitchers faced.
- A hitter can not walk without taking four pitches outside the strike zone.
- The less selective a hitter is about what pitches he swings at, the more likely he is to swing at a pitch he can't drive successfully. This decreases the chances that he will get a hit.
- A single is worth around 0.33 runs, on average, and a walk is worth around 0.25. However, this is balanced against the certainty of a walk when a hitter takes ball four, and the uncertain outcome of a hitter placing a ball in play. If a pitch is likely to be ball four, and the hitter is not in danger of striking out, it is rarely advantageous to swing at the pitch.
Ted Williams' books on hitting constantly stressed plate discipline; Williams told young hitters to not swing at anything with less than two strikes unless they felt certain they could hit a home run. The advice is perhaps overstated, but fundamentally sound. The sabermetric community has done considerable research into the effects of the count on the results of an at-bat, and their conclusions unequivocally indicate that plate discipline is a useful offensive skill even beyond the value of the walks drawn. | 
