Questions about the sport of baseball. The [mlb] tag should also be used if the question concerns Major League Baseball directly.
Baseball is a bat-and-ball sport in a similar family as cricket, with origins in 18th-century England and first played in the United States in 1791. The game as currently known was primarily developed from its original form in the United States, with professional national leagues originating between 1869 and 1876, and has recently spread to other countries including Japan, China, and Central America. The game is played by two teams of nine players each, with some leagues including most professional leagues allowing for substitutions.
The game is played on a purpose-built level field called a "baseball diamond" or simply a "baseball field". Exact measurements vary depending on the age range and skill level (and some measurements can even vary from field to field), but the central feature is a square arrangement of four "bases", spaced 90 feet apart for major league play, with a raised mound in the center. Three bases are square sand- or fiber-filled white bags anchored to the ground numbered counter-clockwise from the right as first, second and third base, while the fourth is a house-shaped white hard plate embedded in the ground, called "home plate". The ground between and around each of the bases is bare dirt, with white chalk "base lines" emanating from home base out to first and third base and then beyond. The central "pitcher's mound" is also bare dirt, elevated ten and a half inches above the level of home plate, with a white hard stripe embedded on it (called the "rubber") exactly 60 feet, 6 inches from home plate in the major leagues. Beyond this "infield" lies the "outfield", bounded on the sides by the "base lines" which continue from home plate beyond the first and third bases, covered primarily with grass and with a fence or wall along its rear border (at a distance to home plate varying between 300 and 450 feet in the majors). At the end of each base line, at the rear fence, is traditionally a pole, called the "foul pole", used to help judge whether a ball hit into play is "fair" or "foul".
The primary and most recognizable pieces of equipment used are the baseball itself, the baseball bat, and the baseball glove. The ball is 9 inches in circumference, roughly spherical, and traditionally constructed of a cork core, wound tightly with thin yarn and skinned in two pieces of white leather stitched together with red lacing. The bat is a club-shaped implement made of hardwood that can be no longer than 42" and no wider than 2.75" at its widest. The glove is worn on the player's non-throwing hand, is made of leather or synthetic fiber, oversized to provide padding and additional catching area, and typically has a web or solid layer of leather between the thumb and forefinger again to assist in catching. Players also wear myriad protective equipment at different times; all players typically wear an athletic cup or pelvic protector as well as athletic cleats. Batters typically wear a hard-shelled helmet that protects the cranium and ear, and often wear a shin guard on the outside of their leading leg and/or a forearm guard on their leading arm to guard against hits. The catcher, a specialized position, uses a more padded glove, and wears shin and knee pads, a chest protector and a padded face mask or face cage.
Play of a game is divided into 9 innings, which are further subdivided into halves, and then into outs. At the start of an inning, one team (typically the visitor) is "batting", and the other team is "fielding". The fielding team occupies various traditional positions around the infield and outfield; typically there is a "baseman" on each base, a "shortstop" behind and to the right of second base (often the second baseman and shortstop take mirror-image positions behind and to each side of second base), three "outfielders" dividing the outfield into "left", "center" and "right field", and the "pitcher" (on the pitcher's mound) and "catcher" (behind home plate) which together are called the "battery".
The batting team sends players one at a time to home plate, where each batter receives a series of "pitches" (throws of the ball) from the fielding team's pitcher, and attempts to hit the ball with their bat so that it flies out into the field of play. A pitch that crosses over the home plate, between the batter's knees and belt buckle, is a "strike"; a batter that receives three strikes has "struck out". A pitch that bounces off the ground or does not pass through this "strike zone" as it crosses the front edge of the home plate is a "ball"; if the pitcher delivers four balls to a batter, the batter "walks" to first base. Regardless of the location of the ball, if the batter swings at a pitch and misses, it is a strike. If a pitch hits the batter's body while the batter is standing in the "batter's box" to the side of home plate, the batter immediately advances to first base. If the batter hits a pitch with the bat and it flies backwards, crosses either base line before reaching the base on that side, or carries in the air to the outside of either foul pole, it is a "foul ball" and counts as a strike (but the third strike must be a "called" or "swinging" strike, not a foul ball) and additionally a foul ball can be caught by a fielder before it hits the ground, and if so the batter is out.
If and when the batter hits the ball into the field between the base lines and/or the foul poles, it is a "fair ball" and the batter attempts to reach first base. To reach it safely, the ball he batted into play must hit the ground in bounds before being caught, and the batter must reach first base before the fielding team can tag first base with a hand or foot while holding the ball. If the ball is caught before it lands, the batter is "caught out"; if the batter fails to reach first base before the fielding team can tag first base while holding the ball, the batter is "forced out". If the batter reaches first base without either of these happening, he is "safe", and becomes a "base runner", attempting to advance from first to second to third base, and then home to score a "run", as subsequent batters put the ball into play. The runner may only try to advance on a batted ball if the batter is not caught out, though if the batter is caught out the runner may "tag up" by touching their current base, and then attempt to advance (this is typically seen when the ball is hit deep to the outfield, and is known as a "sacrifice fly"). Only one runner may occupy a base, and if a runner is on a base and a batter or another runner is advancing to that base after the ball is put in play, the runner must vacate the base and advance, and can be "forced out" if the fielding team simply tags the base the runner must get to. A runner may also, at their option, attempt to "steal" a base by running from their current base to the next base without the ball having been batted into play. This is risky, as the pitcher or catcher can throw the ball to a baseman that can then tag the runner out.
A batter that hits the ball over the rear fence of the outfield advances safely, without fear of being tagged out. If the ball carries over the fence without bouncing, it is a "home run", and the batter and any runners already on base advance to home plate and score (a "grand slam" is a home run hit with the bases "loaded" - all occupied by a runner - and so scores four runs). If the ball bounces at least once and then ends up over the rear fence, that is a "ground rule double"; the batter advances to second base and all runners advance two bases (with any runner on second or third scoring a run). Certain other situations are also called "ground rule doubles", most commonly when a member of the fielding team not in active play or a spectator interferes with a batted ball on the field.
After three players on the batting team have been called out, the half is over, and the teams switch sides; the fielding team comes in to bat, and vice versa. After two halves, the inning is over, and after 9 innings, if one team has scored more runs than the other, they win the game (if the home team, which bats second in each inning, leads at any time after the top of the 9th inning, the game is called at that point even if one or more outs are remaining for the home team; this is known as a "walk-off"). If the score is tied after 9 innings, one or more "extra innings" may be played, until one team scores more runs in their half of the inning than the other team can. There is no "sudden death" in most leagues; both teams will have the same opportunity to score runs in each inning. Youth and adult recreational leagues often do not play extra innings due to time constraints, but major league games cannot end in a tie except in rare circumstances where the outcome of the game will not affect overall standings for either team. Play of the game is untimed, although an informal pace of play is enforced by the umpire; players and coaches may not unduly delay the play of the game. Despite this, games can last over four hours for the normal nine innings of play. The longest game ever played in the major leagues was between the Chicago White Sox and the Milwaukee Brewers in 1984, which required 25 innings to decide and lasted 8 hours and 6 minutes.
Baseball is played primarily by boys and young men in the United States (with girls, women and older men playing the related but slower game of softball), at age levels beginning at 4 years old with "tee ball" (the ball is not pitched to the batter but instead simply placed on a tee above home plate), and progressing through "Little League" (for various age ranges younger though high school), interscholastic high school, college (NCAA), and minor and major professional leagues. The two major professional baseball leagues in the United States are the American League and the National League, which together form the Association of Major League Baseball or MLB. Major League Baseball is the third most popular spectator sport in the United States, behind NFL professional football and NASCAR motor racing. The two leagues of MLB have slightly different rules, the primary one being whether the pitcher must also bat (the National League requires this), or if instead the pitcher may be replaced in the batting order with a "designated hitter" (the American League, as well as most amateur, college and international leagues, allow designated hitters, although if the pitcher wishes it and the manager allows it, the pitcher may bat in any league).
Baseball is also popular in some non-US countries and territories; Japan in particular has embraced the game with the Nippon Professional League, divided into Central and Pacific leagues much like the NL and AL in MLB, and the game also has a devoted following in other Asian countries, and in many Caribbean and Central American territories, which produce many star players in the U.S. major leagues. The game is part of the Pan-American Games, and was formerly an Olympic sport starting in 1904, but was discontinued after the 2008 Beijing Olympics primarily due to a lack of universal following and thus regional dominance by North American and Asian teams. The "Big League World Series", the championship tournament of Little League's high-school-age league structure, is therefore the most geographically diverse baseball tournament currently in operation, attracting teams from every continent. Baseball's parent sport, cricket, is more globally popular, having been introduced to nations of the former British Empire such as Canada, Australia, New Zealand, India, the Virgin Islands, Hong Kong, Egypt, and many African nations.