The strike zone is the area above home plate in which a pitch will be called a strike by the home plate umpire, whether or not the batter swings at it. It is defined in the official Major League Baseball rules as:
- "that area over home plate the upper limit of which is a horizontal line at the midpoint between the top of the shoulders and the top of the uniform pants, and the lower level is a line at the hollow beneath the knee cap. The Strike Zone shall be determined from the batter's stance as the batter is prepared to swing at a pitched ball."
Because the strike zone is defined in relation to the batter, its size will vary according to the batter's height and batting stance (in a crouched stance, such as that used by all-time bases on balls leader Rickey Henderson, the strike zone is smaller than if the batter were standing straight).
The strike zone is supposed to be an objective concept, but some umpires will have a tendency to either call more strikes on high pitches (a "high strike zone") or low ones (a "low strike zone"). Some umpires will give pitchers the benefit of the doubt on pitches at the inside or outside edge of the zone (a "wide strike zone") and others will not (a "narrow strike zone"). With the hope of forcing more consistency among umpires, Major League Baseball introduced the "Questec" system in the early 2000s, an electronic eye that could pinpoint whether a ball was in the strike zone or not when it crossed the plate, and umpires, who still had the only say on actually calling balls and strikes, were judged retroactively based on how much they differed from the machine's judgment. This system was very controversial at first, but it did succeed to some extent in standardizing the strike zone.
MLB has since developed its own system, PITCHf/x, that uses three cameras to track the baseball through the strike zone; it is used for various purposes, including to evaluate strike zone consistency. Many broadcasters have their own less sophisticated systems, usually consisting of an on-screen graphic consisting of a drawing of the strike zone in the shape of a box superimposed on the image of the batter. All of these tend to confirm that major league umpires are extremely consistent in their calling of balls and strikes, but that they are human and thus not perfect. There have been suggestions that baseball move to a fully electronic system to judge balls and strikes, with the home plate umpire left to make calls only on matters such as checked swings, foul balls, plays at home, balks and potential interference calls on stolen base attempts. Such systems were first introduced as an experiment in minor league games in independent ball in 2015.
- Lindsay Berra: "Game Changers: An electronic strike zone? MLB pleased with current system, keeping tabs on new technology", mlb.com, November 30, 2015. 
- The Definition of the strike zone in the oficial MLB rules: definition
- Article on the difference between American League and National League strike zones in the 1940s and 1950s in The Hardball Times