|This article does not cite any references or sources. (December 2009)|
In many team sports, defense or defence (see spelling differences) is the action of preventing an opponent from scoring. The term may also refer to the tactics involved in defense, or a sub-team whose primary responsibility is defense. Similarly, a defense player or defender is a player who is generally charged with preventing the other team's forwards from being able to bear down directly on their own team's goalkeeper. Such intention exist in association football, ice hockey, water polo and many other sports.
By sport 
|This section requires expansion. (December 2010)|
Association Football 
Ice hockey 
In ice hockey, there are normally two defensemen on the ice. One is usually a more offensive player better known for their ability to glean assists or goals rather than for their strong defensive play. Such players are known as offensive defenseman. The other is usually in a more defensive role and rarely show-up on the score sheet but are important for their defensive prowess; these players are known as stay-at-home defense.
Water polo 
In water polo, if an offensive player, such as the center forward, has the ball in front of the goal, and the defender cannot steal the ball, he may intentionally commit a foul. The forward then has a free throw but must pass the ball off to another offensive player, rather than making a shot at the goal. Defensive perimeter players may also intentionally cause a foul and then move toward the goal, away from their attacker, who must take a free throw. This technique, called 'sloughing', allows the defense an opportunity to double-team the center forward and possibly steal the inbound pass. The water polo referee may refrain from declaring a foul caused by a defensive player, if in his judgment this would give the advantage to the defending team. This is known as the Advantage Rule.
Baseball is unlike most other competitive sport in that the defense is given control of the ball. Additionally, the number of players on the field at any given time is lopsided in favor of the defense always has nine players on the field; the offense has between one and four.
Historically, each player on the team had a role both on offense and defense. Most amateur and professional leagues have adopted the designated hitter (DH) rule introduced by the American League of Major League Baseball. In leagues that use the DH, pitchers usually do not bat, and the DH bats in lieu of the pitcher and does not play a defensive position. The main leagues that have not adopted the DH rule are the National League of MLB and the Central League of Nippon Professional Baseball.
Each play starts with the ball in the hands of the pitcher, whose job as a member of the defense is to use his skills to somehow prevent the batter from reaching base. The pitcher throws the ball toward the catcher, whose must catch the pitched ball if it is not hit by the batter. In each half-inning, the defense attempts to force three outs.
There are three basic ways in which an out can occur: 1.) If three strikes are recorded against the batter, 2.) if a ball hit by a batter is caught by a defensive player before it hits the ground, or 3.) if a runner who is between bases or has not reached a base to which he is forced is put out by a defensive player in possession of the ball.
If the batter manages to hit the ball, all nine defensive players become active and use the ball in attempting to prevent the batter from reaching base and runners already on base from advancing or scoring. While the offense is busy attempting to move runners around the baseball diamond toward home plate, the uses the ball in various ways to achieve outs.
If the defence forces three outs, their team is moved into the offensive role. The exception is if it is the ninth or an extra inning and they are ahead, in which case, the game ends and the defensive team wins.