At-bat: A plate appearance by a batter that results in a hit or an out. A plate appearance is not recorded as an at-bat if it results in a walk, hit by pitch, error, or sacrifice. 

Base: One of four square (or, in the case of home plate, pentagonal) rubber cushions at the corners of the infield diamond. Offensive players standing on a base are safe from being tagged out by the defense. 

Base hit: The result of a plate appearance when the batter reaches base safely by hitting the ball into fair territory. (Often shorted to “hit”) 

Baserunner: An offensive player becomes a baserunner as soon as he leaves the batter’s box and attempts to advance around the bases. 

Bases loaded: During an inning, when the offensive team has runners occupying all three bases. 

Bat: The long wooden (in some competitions, metal) object used by an offensive player to hit the ball. 

Ball: A pitch that misses the strike zone. Only recorded as a ball if the batter does not swing. 

Batter: The offensive player who is at the plate, attempting to hit the ball. Also known as a hitter. 

Bullpen: The area of the field where relief pitchers warm up before entering the game. Also used to refer to a team’s corps of relief pitchers. 

Bunt: When a batter, instead of swinging fully at a pitch, holds his bat still in the path of the ball so that the ball travels a short distance into fair territory. Most often used to advance baserunners as a sacrifice, but some speedy players bunt with the intention of beating a fielder’s throw to the base for a hit. 

Changeup: A pitch that is meant to look like a fastball but carries less velocity in order to force the batter into a mistimed swing. 

Clubhouse: A team’s locker room. 

Curveball: A pitch that curves away from its initial straight-line path. A curveball is thrown by exerting rotational force on the ball so that it spins rapidly along its flight path. 

Designated hitter (DH): The offensive player who does not play defense. Used in some competitions (including the Olympics) to prevent pitchers from taking plate appearances. 

Double: A base hit where the batter makes it safely to second base. 

Double play: Any defensive play that results in two outs. Usually involves getting the batter and one baserunner out in quick succession. 

Error:  A botched defensive play that allows the batter to reach base safely on a batted ball that would have resulted in an out if fielded cleanly. 

Extra innings: How ties are broken in baseball. If two teams are tied after nine innings, one or more extra innings will be played as necessary until one team ends the inning ahead. 

Fastball: The simplest and most common pitch in baseball, thrown with high velocity and little movement. 

Fielder: A defensive player (usually not the pitcher, though the pitcher can become a fielder if he attempts to field a batted ball). 

Fly ball: A batted ball that rises high in the air. 

Foul ball: A batted ball that lands outside fair territory. 

Grand slam: A home run that results in four runs for the batter's team. Only possible when the batter comes to bat with the bases loaded. 

Ground ball: A batted ball that bounces in the infield. 

Hit by pitch: When a pitch collides with some part of the batter during a plate appearance. The batter is awarded first base automatically when this occurs. 

Home run: When a batter hits the ball over the outfield fence, allowing him to round all four bases unchallenged. (An inside-the-park home run occurs when the batter hits a ball within the playing field, but far enough away from the fielders that the batter is able to round all four bases safely.) 

Infield: The section of the playing field surrounding the four bases, usually filled in with dirt or clay. 

Infielder: Any member of the defense (not the pitcher) that plays within the infield. Includes the catcher, first baseman, second baseman, short stop, and third baseman. 

Inning: One of nine installments of a baseball game during which both teams take a turn at bat. 

Lineup: The list and order of active offensive players on a team. 

Manager: The head coach of a baseball team. 

No-hitter: A game in which one team does not concede any hits over nine innings. Most impressive when accomplished by one pitcher from start to finish. 

Outfield: The expansive territory of grass in fair territory between the infield and the deepest perimeter of the field (known as the outfield fence). 

Outfielder: Any member of the defense that plays in the outfield. Includes the left fielder, center fielder, and right fielder. 

Pinch hitter: An alternate batter called upon to substitute for a starting batter, often in a critical situation. 

Pitcher: The defensive player responsible for throwing the ball towards the batter to begin play. Generally considered to have the greatest influence on the outcome of a ballgame. 

Pitcher’s mount: The slightly elevated dirt surface in the middle of the infield, from which the pitcher releases the pitch. 

RBI: A run batted in. When a batter puts the ball in play so that a separate baserunner on his team is able to score a run.  

Relief pitcher: An alternate pitcher brought into a game to take over from the previous pitcher. Often shorted to “reliver”. 

Sacrifice: When a batter intentionally makes an out (either by bunt, ground ball, or fly ball) to allow a baserunner on his team to advance to the next base. 

Steal: When a baserunner advances to the next base without the ball being put in play by the batter.  

Strike: When a batter swings at a pitch and misses, or when he does not swing a pitch in the strike zone. 

Strike out: When a batter suffers three strikes in a plate appearance, making an automatic out. 

Strike zone: The imaginary vertical, two-dimensional, rectangular area spanning the width of home plate and the height of the area between the batter’s knees and his center-chest. 

Walk: The result of a plate appearance during which a pitcher throws four balls, allowing the batter to reach first base automatically.