B

Base: One of the four stations on the field that a runner must touch in order to score a run.
Batting Average: The measure of a batter's success in reaching base, calculated by dividing the number of hits by the number of at-bats.
Bullpen: The area in the outfield where relief pitchers warm up before entering the game.

C

Curveball: A breaking pitch that moves downward as it approaches the plate.
Changeup: A slower pitch designed to deceive the batter with a change in speed.

D

Double: A hit that allows the batter to reach second base safely.
Double Play: A defensive play in which two outs are made on the same play.
Dugout: The enclosed area where players on the bench sit during the game.
ERA (Earned Run Average): The measure of a pitcher's effectiveness, calculated by dividing the number of earned runs allowed by the number of innings pitched and multiplying by nine.

F

Fielder's Choice: A play in which the defense chooses to get an out on a runner instead of the batter.
Foul Ball: A batted ball that lands outside the foul lines.

H

Home Run: A hit that allows the batter to circle all four bases and score a run.

I

Inning: A division of the game where each team has a turn at bat and a turn in the field.

O

On Deck: The next batter scheduled to bat.
Out: An action by the defense that results in a batter or baserunner being called out and removed from play.

R

RBI (Runs Batted In): The number of runs a batter is credited with driving in.
Run: A point scored by a baserunner who successfully touches all four bases and returns to home plate.

S

Sacrifice Fly: When a batter hits a fly ball that allows a baserunner to tag up and advance a base after the catch.
Single: A hit that allows the batter to reach first base safely.
Slider: A breaking pitch that moves sideways as it approaches the plate.
Steal: When a baserunner advances to the next base while the pitcher is delivering the ball to home plate.
Strike: A pitch that is in the strike zone and either swung at and missed by the batter or called as a strike by the umpire.
Strikeout: When a batter accumulates three strikes and is called out.
Strike Zone: The area over home plate, between the batter's knees and the midpoint between their shoulders and belt, where a pitch must cross to be called a strike.

U

Umpire: The official responsible for making judgment calls and enforcing the rules of the game.

W

Walk: When a pitcher throws four pitches outside the strike zone, and the batter is awarded first base.